There are two tankless water heaters designed for use in RVs: the Gerard and one made by Precision Temp. The Gerard is lower cost and will fit in place of your old gallon WH without modifying the opening. I saw a demonstration of the unit a few weeks ago and it does put out plenty of hot water for a shower and will keep delivering as long as you have water and propane. However, the way Gerard was able to keep size and cost down was to use a burner with only two fixed settings. To adjust water temperature, one has to use the hot water faucet only (no mixing with cold water) and adjust the water flow. The slower the flow, the hotter the water. To cool the water down, one has to increase the flow. I'm not particularly crazy about that arrangement, especially since I plan on having a clothes washer in the TT I'm going to buy. The Gerard is being installed in many new RVs but the reviews from people who bought them have been less than enthusiastic. The unit works fine as designed but it involves using it in a way that is almost opposite to how one normally would with a conventional water heater, something most people can't grasp or are unwilling to. It will save a considerable amount of propane over a tank unit running on propane. Also, Gerard required the unit be professionally installed. No DYI.
Precision Temps RV500 costs almost twice as much as the Gerard but is more powerful and has a variable gas valve to control water temperature no matter what the incoming water temperature is and no matter what the flow rate is. It does require an opening big enough for a ten gallon tank heater so if you wanted install one in place of a six gallon tank, you would have to do some surgery and reframing to make room for it. I'm seriously considering this one for my new TT (no surgery since the stock water heater is a ten gallon unit.
There is one or two small ones made for use in the shower but I haven't heard anything good about them.
One possible downside to RV tankless water heaters is they run only on propane. They will use considerably less propane than a tank water heater but one does not have the option of running on 110v when hooked up to a fixed rate shore connection.
There are several excellent residential tankless water heaters available but they are not approved for RV use. Doing so voids the warranty and they require far more room to install. They may or may not permit converting to propane.