I have just called the engineer
who fitted my genny/invertor/extra batteries and had a conversation with him about fuel tanks and engines and your problem. And I shall recount it as best I can but please take it in context that it comes from (non-technical) me.
Rod (my engineer) told me that my tanks are separate, and remain so since my genny is on one tank and my heating
on the other.
He told me that if your genny can draw from both tanks then they have a linking fuel line that my boat does not have. I understand that you have a valve that you can push or pull which determines which tank your generator
draws fuel from. Rod told me that this may be fairly complex and may open and close two valves to achieve the switch. If this is faulty or wrongly installed in any way it may provide a clear route
from one fuel tank
to the other.
Rod told me that when a main engine runs it sucks fuel from the fuel tank
, but uses maybe 10% of it and squirts the rest back into the (same) tank. If your generator is linked to this return fuel line, then maybe the return fuel, rather than being drawn from one tank and the excess being returned to that same tank, is finding its way along the genny fuel lines from one tank to to the other. This could happen for many reasons but Rod says that the valve that selects the tank feed for the genny would be his first point of examination.
I asked Rod his opinion given the info that I could communicate but his explanation made sense to me. If you are sure about your observation of one tank only being used then the genny fuel line would seem to be the only route
that the fuel could be following. And if Rod is right about the main engine dumping excess fuel back into the tank then this could be the 'pump' ,that is transferring the diesel
across. And the only route is the genny fuel lines. And since Lagoon
run a 'production line to make these boats, we can assume that there is no chance of them having tried to run any new fuel lines in your boat 'just to see if it works better' like a smaller manufacturer might.
Rod told me that on his own boat, he uses the technique of running an engine and choosing where he directs the excess fuel to transfer fuel from one tank to another. So it must shift a fair amount of diesel
And, it would explain how only one of your tanks is going down and the other is staying full.
I hope this all makes sense, but it is the summary of a much longer conversation, parts
of which were not clear to me!
You may like to speak to a local diesel engineer
to check their opinion on Rod's thoughts.