For $499.99 you can get a 390cc, 6500 Watts Max/5500 Watts Rated Generator with Electric Start
from Harbor Freight. Depends on what you need.
To keep your fuel consumption low, Figure on running the generator at a 1/2 load. So if you add up the stuff you would normally be running at the same time and it comes to 2.5K watts, Then double that amount to find a generator. Be careful as I have seen generators list only their MAX rating prominently. To me, the generator I have linked to above is a 5.5K generator... NOT a 6.5K.
Also these are open frame air cooled generators and many places frown on them because of their noise. You would need to add a car muffler and build a sound reducing cover that will still allow enough air to the unit to keep it from running hot. Unless you will be setting the generator out side of the bus (ours will be built in under the bus), You will need an electric start. Even if ours was outside the bus, I would still have an electric start. I'm getting too old to be pulling a rope start. Don't forget you have to drain/replace the oil far more often than a "real" RV generator. I would think a synthetic oil would stretch that out a bit. This is a thing we really haven't looked into.
Would I buy an open frame contractor generator? You bet! (it's the one I linked to above... if they still have it when I finally get a generator). We won't be running a generator unless we are driving down the road or wallydocking in a parking lot. I figure I can build a sound reducing cover, put a car muffler on it and build a permanent mount Genturi. All of which will go to keeping the noise down (I don't want to listen to it either). I can't see spending big bucks for something that I will use rarely.