If it's prewired for a generator, then where does the prewire run to? Roof rack? Ventilated side compartment? Ventilated rear tack compartment? Under gooseneck? etc.. Go with the gasoline unless you plan to carry alot of LP or don't plan on running it for any length of time.

I have a factory Onan 2400 that is mounted under the gooseneck and the exhaust comes out below at the running board. I have a built in tank and I can run the generator for DAYS and hardly use any gas. I love my built in generator with a built in tank.

I tried to send you a pm about your generator and it will not go through.  Send me the measurements and a price if you do not mind.

I've had pickup box ones, rear tack ones, hay rack ones and a manger mounted one. I like the manger mounted one. BUT next time, I am going to run the exhaust thru to the roof. As now, you can't tie horses on the side of the floor exited exhaust on the manager mounted one. I have never seen this, but should be easy to do...

Pickup box is easy to get stolen, and just a pain to be out in the weather and to close to the sleeping quarters as is the under the noise mounted ones.

The rear tack took up to much valuable tack space. Had issues with stuff falling on generator and tack always smelled like exhaust.

The hay rack mounted one was never serviced/checked as often as needed as to much a pain to crawl up and down. Also was out in the weather more, added more weight, wind issues to roof, and just a pain to take down if ever needed. Yes, it was easy to service if you remembered the right tools to take up the ladder.

The next trailer I get, it will go in the manager with a slide underneath and top exited exhaust.

Fuel tank is in bottom of managers.

As far as propane, ours have always been gas. Lots easier to fill gas tank than mess with bottles IMO.

Good luck!

We have an Onan, mounted on the floor in the rear tack.  There is a screened louvered air intake vent on the door, and an air exit out the bottom of the trailer, in addition the exhaust is routed down and then across the rear to the off side back corner.  The unit is enclosed in an Onan spec aluminum insulated housing, very very quiet.  The 20 fuel tank is gasoline, and mounted in the storage compartment under the manger. You can fill at the gas pump when you fuel up your truck, or add fuel via a gas can if need be.  The remote switch is in the LQ.  We also carry an air compressor under the bottom saddle rack sitting on top of the generator case.  I still carry three saddles on racks, have 2 bridle racks, rake and etc racks, and a brush box in the tack room. I also keep polo wraps, horse hoses and a long multipocket organizer in there. Everything else 9tables, chairs, rugs, BBQ grill, panniers etc is in the 2 storage compartments under the mangers, and the hay rack.  If I ran out of room, I most likely didn't have to have it!  We love this arrangement, have had it for 7 years and still love it.