“My coach does not leak” ….Noe wait, are you talking about right now or tomorrow…. oh, that was last week, I got it. There is one thing you must understand about a GMC motorhome is there will be leaks….. You immediately think, man…. that’s a bummer.
The GMC body was conceptualized, designed, ran past a few big aerospace companies, drug through a wind tunnel, ending being the way any large vehicle like this should be nuilt…… but it cost a fortune and had a tendency to leak. WHY oh WHY whould they do this to meeeeeeeee…….. Remember, in early training films, GM designed this vehicle with a 15 year life span. Yes and where are we now…. pushing 40?
The body of a GMC is not designed to be a rigid structure, just like an aircraft where you can watch the wings “flap” up and down… it has long been known a rigid structure will break but a flexible one will give and ultimately hold more stress and load. The GMC Motorhome body is an amazing design of expansion joints and sealing systems. There has to be plate movement of dissimilar materials needing to be sealed together while leaving wiggle room……… and the systems are 40 YERS OLD!!!!!!!!
Now remember, where there is a penetration there could be a leak and where there is a seam there must be a tight seal. The sealant must have a flexible rate greater than the maximum possible movement of the seam it is sealing. If not, under a bind it will fail. It must adhere but not hold, seal but not be hard.
Yep, sooner or later all systems fail and being the first of it’s design there will always be found unexpected results and that my friends, are the facts and realities of it. You will always need to be vigilant for leaks and for goodness sakes try not to show off and twist the body…. you WILL pop a seam. There is no way around this keeping up with leaks is a constant thing. This is your responsibility to watch for deteriorating sealant, popped panels, damaged gaskets…. these things will happen and that I can guaranty!
Here is a final way to test for leaks…. just like every other test it will not be a definitive affirmation of no leaks but it will show leaks you may not have found any other way……. I call it blowing bubbles….
So you’ve got a flat tire and you can’t see a nail sticking out…… what do ya do? Dunk the tire in a big bucket of water and watch for bubbles…. right?
Take it from me, these things will not float… even though the local GMC club down here takes cruises in the rally season I keep telling them mine won’t float…….. there’s a way to check for leaks like dunking a tire. Take a blower fan…… kinda like this one
and bolt it to a pattern of a window…… flip it up onto the window and plugr in 
Don’t cha just love the portability of the rig and the 2×4 blocks on the top let you use it as a fan on the ground!!!! True southern engineering at it’s finest!
Now add water and soap 
The blower creates a high pressure zone inside the coach, now spread soapy water on the outside of the coach and watch for bubbles from the air finding an opening 
If you have a ladder on your coach, look at where the right side bracket at the top is secured. It will usually cover one of the rear panel screws and may leak. Why, because that’s a corner which is a flex point… the screw holding the ladder pierces the rear panel seal and can cause a leak…… you can see the small bubbles at the top right 
Now you would expect to see air escaping from the brush gasket between the 2 window glasses… we’re looking for frame leaks here 
the bubbles at the center and to the right are the water channel “weep” holes… the one at the top need a little sealant at the seam. 
This leak is actually the “C” channel we cut out when installing our new top/bottom slider window 
($450 each right now)
so that can get sealed. 
Found bubbles from the entry light, door hinges and at the middle of the door. From this test, it looks like we have the leaks for now….. trying to detect them this way…… hey and you get a free wash job for the hours worth of work! What a deal… hey washing 2 elephants takes some work!
I might say “don’t try this at home kids”… you can but you really need some CFM (big blower fan) to do it. Leak tests are weapons to use in the battle…. but there will always be a reason to check over your coach. The sun is killer on sealants…. our flexible body test the strength of sealants every time it moves so be vigilant…
