I haven't done it but the research I have done is that the effort required depends on the tankless water heater selected. If you are looking at the Precision Temp 500 or 550 be prepared to cut the opening in the sidewall about 2 inches larger. If, on the other hand, you are looking at the Attwood tankless water heater it could be a plug and play replacement for the current 6 gallon.
US Army (Ret)
2020 Entegra Accolade 37TS
2019 Jeep Grand Cherokee Trailhawk (Toad)
FMCA - F432054
the DW and I purchased a 2016 22fe in end of November last year. in February I replaced the 6 gallon Atwood water heater with a Truma Aqua Go instant hot water heater. bottom line is LOVE IT.! the installation has to be done by an authorized dealer. it was 1199.00 for the unit and then 500.00 for install. this included a new gas flex hose and cutting and sealing outside trim. fits right into original hole. their company policy is to replace, not diagnose problems. they want the problem unit back at the factory to figure out what caused the failure should there be a problem. easy to drain and you have 2 hot water settings ECO and comfort. I think it is worth looking into. my understanding is that Elkhart factories are starting to switch over to them starting with the B class motorhomes