I’m going to guess here and say that your heater needs “12 inches of clearance” so that it doesn’t overheat in a confined space. If you left the heater out in the open and ran the vents far enough away from the heater, it will prevent the heater from eating and crapping too close to each other. This would alleviate the 12” issue. Another thing to consider is the venting location. If you vent the heater out of the front, vehicle travel throwing snow and ice can clog the vent. I was told that Suburban won’t warranty a vented furnace that is vented out the front because of that issue. I know we might not be comparing apples to apples, but it’s food for thought. Good luck.
My take on Clearances for heating devices is they need clearance to prevent heat buildup for fire prevention. Looking back at the pic I posted with the bunk down, there is 4" of clearance under the wood bunk frame from top of heater. It's like the mantle on a fireplace, different fireplaces and stoves require different distances to anything combustible to prevent fires.
I thought about the vent out the front and thought about a steel cover for it when traveling.
I need to see where it can fit.