The fuel tank on my trailer has a 15min timer. I will look this evening and get some information off of it if I can find any. It is a momentary rocker that I assume feeds a control module. Pulse once to turn on. Pulse twice to turn it off early. I think it would be perfect for you (except for being able to adjust it).
On my dad's trucks we added a led on the dash to indicate when the transfer tank pump was on. Both of us have bathed the side of the truck in diesel before we added the light. The timer would be a lot better. Wonder why I never thought of it...