OK so your generator is an Onan Marquis 5500 watt BGM Spec F generator. The manual I posted a link to should work for your model. You have the electronic governor which makes things a little easier.
You may not need to pull the generator all the way out. Many things can be accessed by simply dropping it to the ground and working over the top. You can assess that once you drop it to the ground. I'd make sure I did have some provisions to be able to lift it high enough to pull it all the way out just in case.
Getting it down is not usually too difficult. Putting it back in does require a little work. The biggest issue I've had with putting them back in is getting the unit and the side screws to line up. Usually if you can get a couple screws in (one on each side) the rest go easier. I've used two floor jacks to jockey it around to get everything lined up. Don't tighten the screws down until you have all of them started. After that it's piece of cake.
The only things that need to be disconnected in order to drop it is the 120V wires, the battery cable and the fuel line. It should just drop down after that. You should disconnect the - battery cable before starting and make sure you don't have shore power plugged in.
While it's out I'd clean the slip rings, replace the plugs, fuel filter and maybe replace the brushes. Depending on the hours and condition you may want to consider replacing the fuel pump. There is a Carter fuel pump that's an exact replacement for the OEM Onan pump. The Onan pump sells for around $230 while the Carter pump sells for around $75. Carter made the pump for Onan. It's a 12V
6psi max pump that is identical to the OEM pump. Part number is P4070.
To clean the slip rings I use a 3M green pad and turn the engine over a few seconds while pressing the green pad down onto the slip rings. You can easily access the slip rings by removing the brushes.
These things are pretty durable and will give you years of reliable service.
Good luck and keep us posted on your progress.