it's a bad design in my opinion, the fuel cap sits below were the water level would be and the vent allows water to enter. I dump some rubbing alcohol in mine from time to time to absorb the water, I have a water seperator in the fule line as well. It's in place of the standard military fuel filter on the front wall.
The other answer is to bypass the internal tanks completely. Two ways come to mind for vehicles that don't burn a full tank between outings. Number one is to plump a flex line to a jerry-can mounted on the backend that snakes into the interior through the cooling grill. Number two is even simple, use outboard motorboat portable tanks. Neither would pass a road safety 12" inspection, but avoid the issues of corrosion and moisture in the tanks.
The fuel fillers on the V100 are caps that look like a barrel bung recessed under a flat 1/4" plate cover. Nothing realistic about their placement either on the front nose slopes. So, the filler and its vent are nestled in a cup below the plate. No wonder rain gets in. Jump ahead to the last production run of the V100, the designers got smart and simplified the caps by getting rid of the recess and going to a stock M113 cast gas cap.