Heat is not an issue since the Honda unit is so well engineered. I didn't find it to be a problem with my box at all BUT
.........vibration and noise are a huge issue! In retrospect they basically ruin the idea of this whole setup. I think a Honda, or any other generator, need to be off away from camp with a long (50'?) extension cord. That way they are silent and the result is deal. I ran the mounted Honda generator on two vans. It is no doubt the quietest one with the least vibration/noise and vibration was bad on both. If you are the type who likes ambient noise to drown out everything else so you can sleep you might like this setup. That mostly describes me but the vibration was still too much.
This setup is completely relative so you almost have to try it to see if it works for you. After much thought I think a solid inverter used while idling the van is a better idea for microwave or heavy AC power use. Off grid air conditioning is elusive.
Also, I had the good fortune to do some testing on another forum member's SMB with a mounted Onan generator with Starcool. Same result. Way too much vibration and noise to use long term. Generators on a big RV work fine but on a smaller van they are overkill.