Thanks for the replies gang.
Bob, when I returned the RV to the dealer, they used tin tape all over the furnace box, and they really did cover the thing to the point of nearly airtight. There may have been an improvement, I hadn't spent enough time with it prior to know.
Jim, that's good to know about the problems with shrinkage. It makes me feel like my quest to solve this problem is worthwhile.
I tried a few things before settling on a solution. I placed thermocouples in strategic places in the cabinet and walls above the furnace. The real trouble was that waste heat was collecting inside the wall between the furnace and the shower as Bob noted. The air inside the wall was in excess of 150 degrees after about 30-45 minutes of continuous run. I have plots of the actual data if you are interested. I stopped the test after 45 minutes as the temperature was continuing to rise. These temperatures are still far from flashpoint, so there was no safety hazard, but relative humidity was waaaaaaay below what wood would enjoy.
So, the problem is tons and tons of waste heat with nowhere for it to go except through the walls. As I mentioned, the wall and countertop were in excess of 140 degrees on the surface, so there was some heat sinking going on, but at the cost of the moisture in the wood. Removing the drawers didn't help either since the heat was rising rapidly and there was no reason for it to go anywhere but up.
After some more data collection I put some small 120mm fans just below the upper cabinet on the splashback wall using a 12V power supply that is triggered by the thermostat. The remarkable thing is that those fans are blowing out 140-150 degree air that was previously just hanging out in the wall. So essentially, I was able to protect the wood from extreme heat and I increased the efficiency of my heater pretty significantly. I don't know how much, but some. Instead of that 150 degree air getting pumped back into the furnace or just heating up my trim, I 'm getting it out into the living space.
I hope this helped someone.
So where are all the RV engineer types living? What's the popular forum for the problem solvers?
Thanks for the tips gang.
Guico