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.
2014 Winnebago 26FWRKS 5th Wheel
2007.5 Dodge 2500 6.7L Diesel
2004 Dodge Durango Hemi 3.55 (Used to tow TT)