If your TC is wired like my older class B's were, yes, plugging it in should give you better starting voltage (Tiger CX starts the Onan off the engine battery). If it starts that way, the issue is most likely either the battery or a loose or corroded wire, or bad 12V ground.
It took me about 2-3 hours to pull the Onan in the Tiger, working alone on a hot humid day. Mine's in a compartment mount, held in by 4 bolts in the bottom. My compartment door frame had to come off to have a big enough opening to pull it.
All the wiring and fuel plumbing is at the back, in the left corner as you face the access panel. Three AC wires, one 12V start, one harness plug for remote start, one fuel connection. Getting those unhooked & re-hooked was the biggest hassle.
I used a 2 X 10 plank as a ramp to get it to the ground. IIRC, the sticker on the side says it weighs 125 lbs.
Jim, "Will adding an exhaust brake make my Onan quieter?"