I had a 2500 msw inverter to start with--but a large battery bank.
The main battery disconnect switch was accessible from the driver's seat. I'd turn on the inverter, turn on the water heater (electric side) and while I was driving, heat up the water. I used 20 minutes "on" and 20 minutes "off". I monitored the voltage at the chassis battery. If it got to 12.9, I'd turn off the heater.
The water heater (10 us gallons) needed 90 minutes on electricity to fully heat the contents, starting from stone bone cold.
Running the water heater on propane is a bit pricey. I tend to do all my cooking on electricity. I had a battery bank of 875 amp-hours. Due to solar charging, almost all trips started with a full battery bank.
On trips of three hours, the water heater would reach cycling temperature.
If no hot water is drawn, the heater will cycle after about 4 hours, and runs for about 15 minutes.
I made two mistakes with the solar. The first mistake was maxing out the charge controller--making increasing the size expensive. The second mistake was buying a generator instead of doing a massive upgrade to solar. I've paid far more for generators than for solar.
