I am sure to uncoil the drain hose when I shove it into the bumper. If you just shove it in, it is coiled and ready to spring, so to speak. I've noticed if I grab the fitting and give it a couple of quick shoves in and out it will uncoil and relax. I bet this uncoiling causes a few of those caps to pop off when going down the road. I have yet to lose a cap either.
2010 Jayflight 25BHS
2003 Chevrolet Silverado 1500HD 6.0L
V-5 Talon hitch
Valley 1200 WD hitch w/ Valley sway