I'm guessing you may already have the generator...
If you don't already have it why not go with a portable self contained unit with a remote start setup. These have built in fuel tanks and you can haul it in the back of your truck and then place it wherever you need it. I use two Honda 2000i with a parallel kit- these were replacements for our Onan that died. They ended up about $800 cheaper than replacing the Onan.
I can use them for the 5th wheel or at home if the power goes out. Lot more flexible than the Onan setup was.
To answer your question- Don't worry about the affect on the hitch weight, if need be you can add weight in the front of the 5th wheel to compensate.