Here is my "on board" generator setup for my 19RR. Generator is a Yamaha EF2400ISHC. This is a 2011 19RR but could be done to just about any trailer if ya wanted.
I did this because just about all the places I camp are atv parks without hookups and I wanted a simple setup without putting a generator on the ground. I also like to be able to pull into rest-stops and start the generator without hooking up cords and such. With this done I can go down the road with the gen on if I wanted, but never have.
Mods to complete the install-
1) I had to cut out the factory angle iron that held the battery box.
2) I welded in new 2"X2" angle to form a square the size of the base of the Yamaha gen to sit in.
3) I built and welded a new battery tray to hang on the side of the trailer tongue, see pic.
4) I purchased a 30amp transfer switch from here- Progressive Dynamics 30 Amp Generator Transfer Switch - Progressive Dynamics PD5110010V - Generator Accessories - Camping World
I installed this behind the fuse panel, one side goes to the factory male plug on the side of the camper and the other side of the switch is a 10g cord similar to the 30' factory cable run permanently along the frame under the trailer up to the generator. This allows me to just start the gen and the switch will grab the gen power when it sees 120v.
5) I had to move the propane bottle rack about 4" forward and use a soft cover because the molded cover now hits the jack. I do not like the soft cover and will revisit this problem.
The only problems-
1) The frame manufacture would have a cow if I wanted warranty work.
2) If the gen is pushing the A/C it is LOUD in the bedroom since the gen is less than a foot from me when sleeping.