Is what happened to me? It's costs me about $12-1500.00 and fuel is around $250-300 depending on how much I leave in the tank.
Coach, I've heard you should always top off a diesel fuel tank at the end of the day. Since it's not a sealed system air can get in and bring humidity with it. Leaving a minimum space for air means less condensation can form in the tank so you get less algae growth.
I cleaned a tank on a generator not too long ago. That black slime is no myth.

Tbh I always wondered why they don't make caps that incorporate fine filters and desiccant since you can't fill a non-stationary tank right to the brim.