How do you install onan generator in RV?

Get a installation manual from the generator OEM. You can get good advive from them.
But this is the way I do it.
1 Double seal all seams and cracks in the box where the genset will be.
2 Cut all holes needed in the floor. For Exhaust,Oil Drain and air flow requirments.
3 Run the fuel line along the frame so it is proteced from the elements as much as possible. Avoid any sharp corners and bends that may cut the line. Do not tie into the main engine supply line. The pressure required for the main engine. Can over power the generator system and cause flooding. BIG FIRE HAZARD. Some tanks have a seperate fitting for generators to pick up fuel. If yours does not you will need to contact a shop that deals with tanks.
4 Run your battery cables the same as your fuel lines. Protect them as much as you can.
5 Some trailers and RVs come generator ready. Meaning that the AC wiring is already at the location for the generator. Then all you need to do is follow the wiring instructions. If yours is not you will need to check with an RV shop to have yours set up.
6 On the exhaust system. You need to be very careful on the install. Do not bring it out under any windows or doors or intake vents. Have some flex in the line for movement to avoid breaking the pipe. Run it out to the section where the fumes can be carried off by the wind. Also good idea to have gas detecktors installed inside RV.
7 Have a door large enough. So that you can work on your set and it can be pulled if it has to come out.