I don't mean to add "fuel to your fire" here but the problem may be even more serious than what you may be thinking. Then again.......it could be a non-issue.
If the furnace does not have enough return air in the fresh air chamber, it not only impedes the performance of the furnace but it
MAY be a serious fire hazard if the furnace doesn't have a high-heat cut-off switch (I don't know if it does or doesn't).
The furnace is still intaking enough
outside air to fuel the combustion of the propane in the combustion chamber of the furnace. That combusted propane is converted to heat (that part of the furnace is obviously working properly). If not enough heat is removed by the circulation of air in the fresh air chamber, I would think that heat may build up to the point of being hazardous and potentially causing a fire, unless the furnace has a high-heat shut-off switch of some sort. I would think that a high-heat cut-off would be essential to this type of furnace in the event that the fan fails.
If it does have a high-heat shut-off switch and the furnace is not shutting down on it's own, your theory may be a mute point, as it would seem that the furnace must be getting enough fresh air to keep the furnace from over-heating, otherwise the heat build-up would surely shut the furnace down (unless the cut-off is inoperable). That would mean that all of the heat that is being produced by the furnace is being released into the living area of the camper. If it isn't.......where is it going? That furnace would have to be getting hotter than hell if there is not enough fresh-air circulation and the furnace is not automatically shutting down.
I'm not trying to minimize the complaint about non-compliance to the installation requirements of the furnace. I just think that the may be a little different than you might be thinking it is.
Just food for thought
.