On my last 5er, which I still have, I installed the See level II sensors. They worked great. They will tell you what % is in the tank. They go on the outside and use the existing wires. I have not installed them on my 38EL as of yet, but will if I ever have to take down under belly.
You can get them in many different tank configurations along with water pump switch. If you have a built-in lpg tank you can even get that on it.
On my old 5er when I would fill the tanks for flushing, I would turn on the sink or shower and let it run. The system would stay on for 5 min at a time and you could watch it go up in 5% increments as it filled.
They where not to hard go install other then dealing with the under belly and insulation. That is why I am waiting in till I have to get in there.
Brian & Terri
Samson (Chocolate Pom)
2017 CC Champagne 38EL
2006 Salem Sport 32 SRV Toy Hauler (still have)