As usual, misinformation in this forum abounds. Forget electric tankless water heaters; you will not have enough power available to run one that puts out more than only enough hot water to brew a cup of tea.
There are several brands of propane fired, tankless water heaters cerified for RVs. Stay away from the Girards. The vast majority of complaints for RV tankless water heaters are for the Girards.
Precision Temp's RV500 has had very few complaints. It will mount in the same space a conventional 10 gl RV water heater does (often, with a bit of trimming on the RV, it can replace a 6 gl RV water heater).
Atwood also has a couple of models that were designed in conjunction with Precision Temp. I haven't heard much about them yet but, then again, I've heard no complaints. From photos I've seen, they are a more compact model of the RV500, using the same components.
Truma is new on the market and I've seen very little information on them.
All of the RV water heaters mentioned are self-venting to the side of the RV where they are mounted on the sidewall.