I used a 1500 watt when I had a PUP and a 13' Scamp buut in my 38' Class A I have 3.5 of them (yes I have half a 1500 watt heater) and it gets a bit cool at night, partially by choice but when the temps drop into the 30's all 4 are running full time.
ON 30 amps you can use 2 provided your batteries are well filled (That is state of charge 90% or more) and you do not use any other electric other than TV's an drelated electronics (In my case 5 computers 2 DVR's 2 TV's and 2 Digital OTA adapters)
I'm on a 30 amp site now 2.5 1500 watt heaters plus a 3rd on a seperate feed
I dod use electric water heater but it is only 375 watts, not 1500 like most are. (I swapped the heat element. element in the tank is 1500 watts at 240 volts. so at 120 volts it is 375 give or take a couple due to heat.)