Your generator as stated is a "Quiet" generator, the contractor's generator is anything but quiet and you wouldn't want it mounted in your MH.
A contractor's generator is basically meant to be used, abused, thrown around, maybe stolen from a job site and then replaced with another one.
I have the same gen set in 2005 and have been very pleased with it. I would have expected yours to last longer. It sounds like you've had it to a shop to be looked at. Since I've never had any problems with mine, I've never priced any repairs on it, but I would get a second estimate on repairs. If you did have an estimate was it from an RV shop that would have to pull it and then send it someplace for repairs or was it from a Cummins/Onan shop?
Bill