Below is a post I made on the Keystone Owners Forum this past February.
If you plan to dry-camp...you probably should not have an on-demand water heater (my opinion). We never dry-camp. Also - Suburban produced a "circulating" unit a few years ago, and those have not worked well (based on forum postings). I don't think you can even buy those anymore.
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I have a Suburban IW60 tankless water heater in our unit and we really like having endless hot water.
You should do your own research on various models...lots of youtube reviews and installation guides.
Girard
Fogatti
Furrion
Suburban
Truma
My observations...Girard, Fogatti, Suburban, Furrion generally cost ~$600 for the unit and you can install them...it is not a very difficult job.
The Truma costs ~$1200-$1300 for the unit and they require installation be performed by Truma certified installer. I would guess the installation cost to be based on 4 hours x RV Tech hourly rate $150??? (purely my guess).
As you do your research on various models - pay attention to the BTUs produced by each and the required annual maintenance/ descaling proceedure. I assume the BTU rating is related to how quickly the flowing water can be heated. The IW60 is a 60,000 BTU unit. My IW60 does not have descaling hookups built in. I still need to add those valves and connections which I can buy the pieces and parts at Home Depot. The Truma has a built-in descaling process to perform annually (nice!). I can not speak to the descaling requirements for the other models. The pic below shows the decaling piping for my IW60. Basically - I will pump descaling solution (vinegar?) circulating thru the internal heater pipes for an hour or so. I need to add valves to keep the solution from running into the RV piping system. The pump and the 5 gal bucket of solution sit out side the RV, and I will hook up to new connections I install in the basement. The pump can be a 12 volt pump or a 110 v pump - whatever your preference. I figure the descaling stuff will cost me between $200 - $300 to purchase...the pump is the largest part of that cost.
The Truma has a small heating/surge tank built in. The water can be kept hot in the tank so that when you turn on hot water - you quickly get the heated water from the surge tank. On my Suburban - you turn the water on and you wait 10-15 seconds for the hot water to get delivered to your shower or faucet. The Suburban does not "turn-on" until it senses water flow. We set out water temp to 130* (the maximum) and temper with cold as needed at the faucet. In our unit the heated water temps are affected by the temperature of the water coming out of the ground from the City outlet. The colder the water supply is - the temp at the outlet will be less than the desired 130*. I can't speak to this issue for the other models. I turn on the shower and stand outside while I wait for the hot water + get it regulated to suit me - then step in. I skip the "blast of cold water" others have experienced. Also - we do not boondock - ever! I am always hooked up to 50 amp campground power. I can not speak to how well these things are suited for boondocking.
The second screen shot is from a post I did on the Montana Owners Forum showing actual temps of my heated water at each faucet/outlet.
Here are links to other forum discussions on the subject.
https://www.montanaowners.com/forums...ad.php?t=85412
https://www.montanaowners.com/forums...ad.php?t=86790