I believe that the QD 3200 is no longer made, thus limited parts. I bet the core engine is still good, but the other external stuff is probably at the end of its life.
Look on Norwall's website at https://www.norwall.com/categories/RV-Generators/
. They are one of the biggest if not the biggest dealer/distributors of Onan RV generators. They don't have the QD3200 listed, only the QD6000 which probably won't fit and has much more power than you will ever need.
The only reasonable replacement for a diesel powered MH is an LPG powered one. Onan makes a new inverter based propane fueled 2500 model. If you can get by with 2,500 watts then that is the one I would replace yours with. You may have to add a soft start kit for it to start your A/C.
If you need more power then the Onan 3600 LP should work but it is not inverter based. Sooner or later Cummins should come out with an inverter based version. If you can I would wait for the inverter based model. It is more efficient and less noisy.
I don't know of any other manufacturer of air cooled, compact RV style generators.
Edit: Cretivepart and I did a good job covering this question together, right ;-).