I have seen a SeeLevel guage on a Prevost coach I was looking at when shopping several years ago. As I remember it was a pretty simple system, with an electrical contact which was taped/attached to the outside of the tank, and I guess sensed the level via temperature or ??

Anyway, it worked fine. Instead of like some systems where you get 1/4 tank increments, this had a digital gauge which displayed in percentage.

I can't give you any idea of reliability, but the seller was really happy with the system.

Ray