First, you shouldn't get the issue of your fridge not operating as efficiently when your water heater is on. Maybe have something looked at. Now to answer your questions.
Keeping the water heater on during the whole time out really comes down to personal preference. That preference can vary with outside weather climate. Personally I turn my water heater on when I first get to a site and then turn it off during the day turning it back in the evening. My unit maintains a good hot temperature throughout. Now when the weather gets colder, I leave it on.
I am not aware that your fridge would suffer when the propane is low and you are sharing with the water heater.
Definitely, cool your fridge on shore power prior to leaving on a trip. There are lots of arguments for an against leaving propane items like your fridge and water heater running while under way. Some states have laws which say you cannot have your propane bottle in the open position while under way. Some state say propane must be turned off in tunnels.
Propane guages are handy, getting twin tanks would be a nice upgrade.
I see you are in Stagecoach, NV. I am from Dayton originally my father is still there. I have settled and retired in Alaska. However, if you want a good honest no BS person to look at your rig (if its not under warranty) to double check if there may be a problem with your system; go check out Ed's RV repair in Mound House. Some people have issues with him because he seems distant and acts put out. He is the type of business who will tell you what it cost to fix a problem then if you ask he will tell you how to fix it yourself and how to save money doing it.
2015 White Hawk 25BHS Glacier Package
2004 Dodge Ram 2500, 5.7 Hemi, 5Spd Manual
2008 Arctic Cat M1000 SnoPro
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