Heat shields and insulation
I am not familiar with your type of motorhome, but I have dismanted alot of vehicles in my day. (I used to own a junkyard)
If you do not mind getting dirty, find a salvage yard that has trucks and buses and find a heat shield first. This is an aluminum looking metal shield that can be placed between your muffler and storage bin. If your rig already has one, consider replacing it with a larger one.
While at the yard investigate the trunks of a few vehicles. Most cars have thicker and heavier insulation if the muffler gets close to the trunk. Keep in mind, many cars have mufflers in different areas, so best to find a car with the muffler in the back so its easier to see and pull out some mat style insulation.
A third option is to figure out a way to force more areflow between the muffler and the compartment. Without crawiling under your rig, it would be hard for me to suggest a design.
Lastly, if all else fails, maybe provide air ventilation holes (screened of course) in the compartment.
Just my 4 cents!
Ted & Theresa 'TnT' and the cats
Easton, PA 2000 Bounder DP 39z