Ours was the cap on the top of the water pump coming loose. It unscrews so a metal screen can be replaced. Also had to reseal the threads on both the inlet and outlet fittings of the hot water tank after the first season. As suggested - it's on a check list, though I am currently checking several times over the course of this season. Fortunately both are very easy to get to.
TT: 2015 Jay Flight SLX 195RB Baja Edition
TV: 2014 RAM 1500 Bighorn CC, 5.7L Hemi, 3.21, Factory Air Suspension/Tow Pkg, Andersen WD/SC