I just discovered J-B Weld WaterWeld Epoxy Putty, and it's something that should be in every RVer's toolbox for fixing water leaks.

WaterWeld comes as a stick of putty-like material, with the two components separated. You cut off the amount you need, knead it well to mix the two components, and then press it firmly into the leaking area. One hour later, it's hardened into a permanent repair. 

My rig's plumbing fittings are mostly plastic, and they seem prone to cracking and leaking. I just used WaterWeld to repair a leak in the (plastic) water heater bypass valve, and it worked great. I'm out in the boonies of Colorado, so it would have been a pain to get a replacement and then install it. I also had a cracked check valve on the water heater some months ago, and ended up having to quickly find and install a (brass) replacement--I wish I'd known about WaterWeld at the time!

The directions say you can even use it under water, but just to be safe, I turned off the water to remove the pressure and dried the area before applying the putty.

You can get WaterWeld at places like Home Depot or Lowe's, or from Amazon. I know I'll keep some on hand for future emergencies. Hope this helps someone else!