I assume the vent pipe does not have to follow a purely vertical route? I'm thinking of routing mine up a couple inches to the bottom of the floor, then across to the side of the bus, drill a hole there and have it poke through the skirt and then run up the side of the bus to vent at the roof level. This way the only hole I have is in the side of the skirt. Granted, the vent pipe will be visible running up the side of the bus, but... ooh wait, I just had an idea
... I will need a ladder on the back of the bus for roof access anyway. I've been thinking of making it out of black iron pipe. I could connect my tank vent to the bottom of one of the ladder's vertical pipes and send the gas up that way through the ladder!