As for leaving the coach plugged in 24/7 that is fine and yes other than the electricity it is fine to leave refrigerator plugged in however make sure you are level. If it is not level than leave the refrigerator off.
As for the hot water heater you should not leave the plug out after you drain it as insects may go in there. I drain my hot water heater everytime I get home from a trip and then close it up and I also drain the fresh water tank too.
Winteerizing is another discussion.
Hope this answers your question
2014 Winnebago Adventurer 35P,Ford F-53, V-10
2011 Ford Escape,2000 Roadmaster Tow Dolly
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