I agree with the truck bed if possible but I have mounted a genny on the rear bumper of a TT. It needs too be good and strong as it gets a slamming effect or whipping effect. I made one for a generac 3500/4000 remote start propane conversion. I boxed it in and had a 12 inch axial fan from Grainger that started up as soon as the generator fired up. I used a rigid foam called thermax to line the enclosure as it has a high fire resistance and is often code for crawl spaces and basements. Problem was that I dont think it worked that well to dampen the sound because of the rigid nature. So over all I would never convert one to propane. Gas is easy and sold on every street corner.
I now have a 5er and mounted a 3000 Honda remote start in the truck bed in front of the hitch. It's bolted to the bed and has a 12 gauge metal enclosure with a lockable lid. I used the same 12 inch fan to ventilate it and used wavey foam panels glued inside that are about 1 to 1 1/2 inches thick. It's the quietest Honda and the enclosure helped even more. if I park the truck with the hood to the 5er hitch you just about cant hear it inside.
One of the things that happened with the TT bumper mount was a certain amount of vibration or harmonics by being attached to the TT. I used the original rubber mounts from the Champion generator but I think it was an issue. It depends on how you want to use it too. if you need it for rest stops in warm weather or battery charging for limited runs, it will be fine. If you are going to use it a lot I would shoot for ultra quiet. I'm figuring out a way to make a small hoop type easily removable foam "tent" to put over mine to quiet it even more. I have had people say they didn't even know I was running a generator until they walked up close to the truck. I'm just going to come right out and say it......I love my Honda. :)
Similar to thisGutted Champion
Still needed to build a lid and add filler panels