Ned said:
All that says they aren't too practical in an RV

The standard gas/electric water heater just works, and works as you expect it to.
Again, not necessarily. The RV500 and the Atwoods are used the same as a tank type water heater; one blends hot and cold water to adjust water temperature. Also, the water temperature of a tankless will stay consistant while using instead of gradually getting cooler as it is used.
Despite the quirks of the Gerard, many people will be willing to put up with them to get endless hot water at a lower price. I'm not one of them and would prefer the more expensive of the two Atwoods or even an RV500. Most complaints I've seen on the Gerard had to do with the owner not using it properly or the installer not ensuring adequate flow (such as removing flow restrictors, upgrading the pumnp if too slow, etc.). A couple have to do with cold weather operation; the two speed burner keeps hunting at certain temperatures, causing fluctuations in water temperature. I've seen more reports from owners happy with its operation.
I've heard only two complaints on the RV500. One was by a clown who bought his used from a mobile repairman who claimed it came from an RV owned by an elderly woman who was frightened by the sound of the flame coming on (if that wan't a phony story...). The other said his worked fine other than having to have a board replaced a couple of times. I've seen numerous reports of satisfaction with the RV500. Gary Bunzer, the RV doctor, tested the unit and gave it a good report.
Not everyone will consider a 6 gl tank type water heater to be adequate (including me). Some people want longer showers than you. Some, like me, need a longer shower just to get the kinks out in the morning. There is no way any more than one or two people will get enough water from a 6 gl. tank type water heater.