I have a couple of guesses, but I also have questions.
Do you have an additional fuel filler on the MH?
Have you seen a second fuel tank under the MH?
I know in the GMC FWD Motorhomes they had a dual tank setup that was filled from a common filler tube. The switch may be to switch to the "aux" tank and it is full unless you have less than ___ in your main tank. The only way to know for sure is to start taking stuff apart and tracing wires and hoses. I have not see one that looks like that before.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato