Ok, so you wanna know about fuel injection as relating those systems to the GMC Motorhome. Ok, I’ll tell you how I feel about the subject but understand this is only my opinion and living in America as we do everyone has an opinion— some people have more!
Above you are looking at an Electromotive Tec IIIr direct port system, very exotic and high tech. When properly set up, it runs great. There are many other systems that can be installed (both direct port and throttle body styles) that will adapt nicely to the 403 ans 455 GMC motors. Hey, leave it to the aftermarket and hobby guys to provide us with as much stuff as you can stand when it comes to upgrades and performance stuff.
But wait a minute, is that what we want. Do we want to have a performance hot rod running around with exotic parts being relied upon. Do we want to have to call special people and have parts “GROWN” to get us out of a jam when something breaks? Do we want to not have a motor the mechanic at “Bubba’s auto emporium” has ever worked on or systems he has never seen? I have installed many different fuel injection systems, followed customers all over the country answering questions, aiding in diagnosis then having parts over night shipped to support the repairs, this is not right. You should not have to hope your cell phone works and UPS delivers to some out of the way place to get vital parts.
Granted the way new cars have such a low maintenance need is because of computer engine management systems, fuel injection (mainly direct port these days) and “plug-n-play” replacement off shelf parts but putting these components on an old motor can create a “morphodite” machine that no one will work on. It’s a breeze to work on something you are familiar with even over something that is supposed to be a higher technology, more performance system. After what I have been through supporting customers issues, I would gladly give away some performance for ease of maintenance and field serviceability any time. Had one customer I cherish break down for 2 days on the west coast spending buckets of money trying to get his fuel injection system fixed and when it turned out the entire problem was a corroded connector that the manufacturer said should have never been installed in the exposed position it was– I felt terrible– hey, I did not chose to put the customer in harms way, someone else did that and did not tell anyone that they were putting people at risk of failure. These are the sort of risks and “unexpected results” you have when systems are modified.
So how does this impact the concept of adding fuel injection to our Q-jet driven 455 and 403 motors? Simple, any modification you do to your motor or coach in general for that matter must be supported both with technology, engineering and parts by whoever supplied that part. In other words, your motor say is only as reliable as the company that produces the fuel injection system you have purchased. Was the system designed for and tested directly for your application or was all of that borrowed say from a 454 engine set up? Can the parts and even the diagnosis of the system be had anywhere you may break down? Can anyone understand, diagnose and repair any part of an aftermarket fuel injection system you install on your coach? Let me answer these questions all with one word– NO! Break down in the middle of nowheresville and see how they get you going. Sometimes the only parts store you can find is a livery stable with once a week delivery from a catalog. Sometimes the only way to get a part out there on the brave frontier is by custom 4 part delivery at a high price. Then lets not even talk about possible tech help out there, you just cannot take the risk that someone will understand the computer controlled system that has just failed and left you stranded. No, you cannot call that an improvement on the reliability of your coach. What good is new technology if when it fails there are no alternatives to having the coach towed— every time!
I said all of this to say this– if you are your own technician, have the knowledge to repair the systems of your coach, you have the tools and the replacement components you will need that cannot be locally found then adding these exotic components to your coach is fine– have at it! But if you do not want to be driver and head technician– want to drive your coach and when there are troubles toss the keys at a tech and say “fix it” then you do not want to include fuel injection of any sort on your GMC. At least not if you want the machine serviced anywhere you stop. I know this flies in the face of many out there that have fuel injection systems and are very happy with it but I think that means you understand the system and are weighing the risk to make that decision— or you are very lucky and jumping up and down on a thin limb up in a tall tree!
I will continue to install fuel injection systems for folks that understand and can work on their coach but will not recommend FI systems for those that just want a reliable means of transportation. The Q-jet carb is a pretty good system if it is properly maintained. Maintenance of carbs is higher than a FI system but following it’s needs, a carb will give you good performance and that’s where I think most of us want to be.
So stay with FI if you have it, understand it and like to have it , stay with the carb system if you want to be fixed on the road anywhere. I hope this answered your questions and did not upset you. I have many customers out there with fuel injection which are happy with the systems, we have done our best to support them and will continue to do so. If you would like to add FI to your coach, I feel with the experience we have had with it I can give you some good recommendations as to systems to use and where to get it. Let me know if I can help you with that. I have had to come to this realization after years of working hard for customers, supporting products and solving problems both in the field and here at the shop. If you have a contrasting opinion of FI systems, you have that right just as I have the right to see them as I do. Actually, the biggest factor that made me come to this decision was made by someone deep in the FI community who has a business of installing fuel injection systems exclusively. He told me for on the road security, FI systems should not be added to a coach like a GMC. They are too big to push, they do not fit well on a trailer and you never know what assets you will have when you break down out on the road. An exotic FI system is the wrong thing to try and repair out on the road. These are the words of someone in the FI industry– and I agree. KISS, keep your engine simple and you can keep it running.
Of course as I said before, if you like the idea of having that new technology, have the expertise to be your own tech support, have at it and feel good about having this new stuff — all that’s a good thing for some– but for others it can be quite the problem.
Contrasting comments on this subject are welcome and I will post your thoughts if you like me to. 
Why do I always do this to myself!!