Darrel,
IMHO the 2800w unit should be more than enough for your coach, unless you have added considerably more AC loads. The 3000w unit may be overkill and not sure you are getting needed features for the extra $250-$350. We have a residential reefer and all electric coach and our 2000w Magnum does just fine, although I wish it were a pure sine wave unit. We do have to run the geni when using the microwave but that is not a hassle for us. The 2800w should be able to handle it. Also keep in mind that the higher draw from the inverter also means a higher draw on your batteries.
Steve