Jim, Beyond eBay & CraigsList, the only thing I can suggest for a used one is to call or send an email to the service department managers of various RV dealers in your area.  
As far as a good tech goes, I used the then-new Cummins-Onan distributor/service center in Spartanburg SC when the Tiger's 2800 failed under warranty in 2006. I can't really recommend that route. I had to return to have a vital bolt in the governor linkage tightened about a month later. The original tech was no longer there. Years later when replacing the fuel pump, I found that he had also left out a vibration isolator the 1st time.  They mailed it to me free, with no more proof than a phone call & an emailed photo. It didn't really change the noise & vibration, but it still left me with a bad taste. 
 I'll be glad to help you troubleshoot it if you want to try. There are 3 pretty simple things that are common failure points. None cost over $250-300 and a half-day of DIY work, tho' you do need to drop the box to get to at least one of them.  
Jim, "Blessed is the end user who expects nothing, for he shall not be disappointed."