I personally wouldn't use the cupboard.
After doing a bit of cold weather camping with a cube heater and now a Mr Buddy propane
heater, the dynamics of the cube heater barely keeps up with the space on cold nites and putting it inside something would probably reduce the efficiency by quite a bit. I wouldn't want to give up the premium storage space either.
Like Donna, we set the cube heater on the stove top cover which is a great location with four of us in the 13. Haven't had any incidents with it up there, but we don't cook inside and do all the cooking outside under the 10x10 awning
It would seem that if you have the cupboard space then you could go with an Atwood furnace
or some such that is permanently installed. I don't hear of any problems with the dog folks that have the furnaces, and then it would be on a thermostat as well.