I have an tc800 and have found that the manufacturer is responsive and helpful. They'll tell you where to put a screw or what that weird wire is if you call them or email them. (They are also willing to do some customization if you're ordering from scratch.)
We don't have the cassette toilet or shower setup that the 850 has but the rest is similar. Everything seems to work fine and it's pretty simple and mechanical stuff is accessible.
We did have an issue with the battery charging. The dealer walked us through trouble shooting over the phone because we didn't want to drive it 2 hrs to the shop but eventually we did have to bring it in. The harness plug thingy needed replacing. You'll probably have a closer dealer. :)
The 850sc is really nice. We just needed the bigger dinette more than the indoor toilet. We're classy like that. :P