I would shop around for another shop and take it there even if it meant towing it. We replaced all six injector cups on our 350 Cat a year and a half ago when it would momentarily go into limp mode throwing a "low coolant" error. I'm not sure they were all bad, but we made the decision to replace them all since they had the engine torn down. The bill was about $3000, but they accepted our extended warranty so we paid $1500. That bill also included yearly maintenance.
It is a b***h getting good work done on our diesel motorhomes. Too many shops don't even want to mess with them. And BTW, we had taken our Phaeton to 5 other garages, all capable of diagnosing and fixing it before we found the one who did the work. That is where we have all our service work done now. When we walked in and I started to describe what was happening, the two guys in the office started finishing my sentences. They knew exactly what the problem was from what little I told them. So there are decent shops out there. . . you just have to find one.
Hope you get some help with it. Certainly takes the fun out of RVing, but there will be someone out there who knows what they are doing.
Dale