Several possibilities come to mind. First, when in the van, you might
not be getting enough draw on the fresh air (it takes a while to heat
up the exhaust flue so as to ensure that there is a nice draw on the
fresh air side.)

It is also possible that the intake is blocked when you have it
installed in the van.

Finally, you make have built up ash in the combustion chamber (as I
did). It takes only a moment to unscrew the chamber with the fridge
out) and get rid of the ash (I just used some canned air).