I agree with the previous comments. I don't think you can go wrong with either Colton Truck Terminal or Redlands Truck and RV.
Yes, they are both in the San Bernardino area so not in Los Angeles proper but I think are worth making the trip ...a bit over an hour away from downtown L.A. Places in L.A. and its suburbs are usually difficult to navigate, sometimes having to fight heavy traffic to get to, and don't have the established great reputations as the two mentioned.
I've used both Redlands and Colton and both take good care of motorhome customers. Many times truck repair places are focused on their fleet and Class8 clientele and do not treat motorhome owners with respect. My tendency would favor Colton because they may have slightly lower prices but probably not by much.
Colton Truck Terminal Garage -----> website
go HERE
RV service reviews
Google reviews
Yelp reviews
Redlands Truck and RV -----> website
go HERE
RV service reviews
Google reviews
Yelp reviews
You'll also find lots of comments on both if you do a search on this forum. Redlands Truck and RV used to be an iRV2 forum sponsor and the manager used to participate on the forums but I haven't seen him around for awhile now.