Water Heaters, furnaces, and Air Conditioners all fall into a category of you best do it right from the beginning. Or you will spend more in the long run.
I have gone from one extreme to the other. Wasting money on everything along the way. I believe there is a reason commercial RV builders select certain major component. No….. not because they are trying to go cheap. Quite the contrary.
I believe the driving factor has to do with liability issues relating to safety. Secondly, avoiding service issues or component failures during warranty periods.
For the money you really can’t beat the modern LP tank type water heaters. The type with 12VDC controls and electronic igniters. Installed properly, and functioning properly there is little chance of carbon monoxide poisoning. Plus you will have convenient and reliable hot water in all camping scenarios. Whether you have park power or are boondocking.
There are even dual mode units. LP in one position. A 110VAC electrical heating element in a second position. This dual unit may be attractive to someone that predominately has a park power hook up. I personally dislike the 110VAC element when used in a small space. They seem to make wierd noises. Though some people don’t like the start up noise of the LP burner either.
I have tried on demand portable water heaters Yes they will provide warm to hot water. But it is often not convenient without a permanent installation. You have to hook up and disconnect hoses. Worse you try to hang the unit in a confined small living space that is exhausting carbon monoxide, while at the same time consuming oxygen. Though those levels may be slight, that hazard is real. Something to consider before you try to save a few bucks cutting corners on your water heater selection.