I just read specs from a company selling Onan 2800 gasoline generators, and they claim it is pure sine wave, so you should have clean power.
Is your microwave built into a cabinet, and if so does it have plenty of ventilation around it? Some of the microwaves built into RVs, including mine, have less than optimal (cheap and small) trim kits. Especially with a convection microwave, you' really need a lot of air circulation, otherwise it's going to overheat and shut down. I just have a basic microwave in mine, so I don't worry about the cheap trim kit. Microwaves like yours actually have factory trim kits that you're supposed to use, and they do make sure you have plenty of room around the microwave, and enough air circulation.