Be careful when playing with 110 VAC ciricuits unless you are qualified. That said I just had to repair my microwave and discovered that the cheap RV dedicated outlet above the microwave had very badly burnt and corroded internal knife-blade terminal that were obviously heating up and creating circuit resistance which would drop the voltage being recvied by the microwave.
I would have someon checkout your micorwave outlet and make sure it's not heating up and failing internally.
A telltale sign of this happening would be discolouration and pitting on the microwave power cord plug spades. Have a quick look att hem for discolouration and pitting.
Secondly your circuit breaker could be heating up and drawing down the voltage arriving at the microwave as well.
In your driveway your supplied AC power could be marginally higher than at campgrounds and that's why you are seeing it happen.
try plugging the microwave in a different outlet of the trailer while at a campground or plug the microwave directly into a 15amp outlet on a campground power pedestal and see what happens. This would rule in or out your trailer as a contributing factor.