I would not run them with a tonneau cover closed over them. Consult the instruction manual, they advise how much open space is required around the generator for safe operation. There's an air intake on one side and exhaust and air vent on the other side. When you look inside for an oil change, you can see everything is crammed real tight, and then heavily insulated to keep noise down. Open-air ventilation is key to a long life for these great generators.
Running them in the bed will surely make it louder, the bed will pick up the vibrations and rebroadcast them. If you're driving on the highway, you won't hear it, but if you're parked at a campground, especially a forest service or boondocking in the woods, you'll hear it real good.
I have a large toolbox behind my cab, and my twin Yamaha sit just behind the tailgate, with an expensive chain and cheap lock to the small bed tie down that's in the corner.
If I'm stopping somewhere for a quick overnight, I can open the tailgate a little, resting it on my tongue jack, and aim the generator exhaust toward the center of the truck bed. I've only ever run one at a time in the bed.
If I'm stopped somewhere for a few days, I can pull them out sit them behind the truck, with exhaust pointed toward the truck, away from the trailer. The tailgate gives a little shade and the chain's long enough I don't have to unlock anything.
Have fun on the maiden voyage!
2013 KZ Sportsmen Classic 200, 20 ft TT
2020 RAM 1500, 5.7 4x4, 8 speed