I think yours is actually a 2500, not a 2800? Anyhoo, if I'm reading it right, it's started off the camper 'house' battery, the camper has been in storage for some time, and you started it several times. I'd start with suspecting the battery.
Unfortunately judging by my three gas 2800 Microlites, there's no way to get to the starter or solenoid with a meter or jumpers short of pulling the top off of the box. That usually means the generator has to come out.
Jim, "Even if the voices aren't real, they have some pretty good ideas."