IS this the same issue about your pop-up ?
https://www.irv2.com/forums/f54/batt...ml#post6062309
AS for water heater.......
Brand/Model of Water Heater??
Shorted element/wiring would cause a GFCI to trip due to 'leakage' of current onto the ground circuit
*element doesn't have to have FAILED just enough current to ground to cause imbalance between hot/neutral
Fridge......pop-up typically have small 4 cu ft fridge that runs on propane or AC
Some are 3 way...use DC element---they really put a load on Battery. Not to be used for cooling unless battery is being charged/maintained IE: traveling/tow vehicle alternator
(NOT when stationary w/o being hooked up)