I live in Phoenix and store our trailer in a covered facility. I've never thought of putting water jugs while stored. I assume to keep the humidity up so that woods and other interior parts do not dry out and warp, crack etc. With 4 trailers over 25 years I've never had a problem.
We do not keep canned foods, soda or perishables of any kind in the trailer. Toothpaste, soap, cleaners etc are but so far no trouble. Did have solid deodorant sticks melt a bit. Doesn't seem to get frying hot inside even in the heat of the summer. Shade really helps keep the trailer looking nice for years.