IMHO: Tankless water heaters in any RV are a very poor idea. Unless you have full hookups 100 per cent of the time they will not provide the heavily advertised "endless supply of hot water" {what a joke}. As noted using a Tankless system will quickly fill your gray tanks as it empties your fresh water supply {and your propane tank}.
Keep in mind most of the tankless systems do not work very well as they are extremely temperature sensitive which can vary from exciting to downright painfull with the instant temperature swings. Our 6 gallon AC/LP water heater is enough for both of us to have a decent short shower and if you can wait just a few minutes between showers you will have all the controlled hot water you need.
As always... Opinions and YMMV.
:C