While most RV manufacturers do seem to find the lowest price junk appliances for their high dollar units, one of the contributing factors is they don't typically follow the installation guidelines. For a microwave that is built into a cabinet it needs an add-on vent shroud and I suspect the OP's unit probably doesn't have that or it is not installed properly. Even a cheap microwave/convection unit should work relatively well if installed correctly.
I've had them in all my RVs and never had problems with them working the way they're supposed to. My problem with them is being they're low quality little things break on them rendering them only semi-useful over time. But when they worked I was able to do things like bake bread in them.