2010 R29 main tank auto siphoning into auxiliary tank

Long story, short version: I discovered diesel has been leaving the main/forward tank and filling the rear/auxiliary/Wallas heater tank (that’s not working). My assumption is when the bow goes up the fuel flows by gravity back to the auxiliary. The Johnson fuel pump does not have a back flow prevention valve or what not. It does pump from auxiliary to main.

So…. install a simple valve in the fuel line or a backflow valve? Install a new pump that has a backflow prevention system (if exists)?

Keep your selector switch in the off or generator position when not transfering fuel. That should stop any siphoning back to the aux fuel tank.

Are you referring to this switch? I was informed it is disabled and no longer a switch between shore power / generator / whatever.
My fuel pump that feeds from the auxiliary tank is on a rocker switch right next to the pump under the starboard hinged step area. I’ve used it. You have to hold it to pump. I had the tank 50% emptied into the main and then it was full again after a trip out.
I still don’t have a grasp of what switches are inert, repurposed, or as expected. Clearly some modifications have been done since 2010. This whole boat is different from any boat I’ve ever owned before.