You can check the vent for leaks by removing the chrome vent cover on the
exterior of the Vanagon. Place a cork from a chemistry set(or similar)in
the hole that is furthest in. Make sure that the  little metal cap is
screwed in tight at the front of the frig.Then just blow into the remaining
vent on the exterior of the van.It should be a tight seal. Maybe just a
tiny bit of leakage is ok(I got this). A broken corrigated vent pipe is
usually caused by removeing and replacing the frig. Also by adjusting the
aluminum vent collector at the exit of the vanagon.Basically you have to
flex the pipe to break it. Have you done this? Other leaks are the rubber
gasket that seals the top and bottom pieces of the burner.No way has been
found by science to replace that gasket,without first removing the frig
from the van .