I have been using the RV-500 for 6 years. The first 5 were in a 1998 Monaco Dynasty MoHo. It was the first unit of it's type that Monaco ever installed in one of their coaches. It worked flawlessly until I sold the coach last year.
When I bought my 19CCD in Dec. and looked at the 6 gal Atwood water heater, I realized that a 3 minute shower would not do for me. I installed another RV-500 in the CCD also. It WAS NOT a direct replacement, but took a little 'coaxing'
to get it to fit. This included trimming 2" from the top skins and adding 4" aluminum strips to the sides to create the proper sized opening. The rest was a piece of cake!
They WILL fit right into a 10 gal. opening, but most trailers come with a 6 gal unit. It's no big deal if you are careful when you cut into the inner and outer skins.
I am also a dealer for Precision-Temp and offer the RV-500 at a 15% discount to Forum members.