The reviews I looked at for General RV, Birch Run are over 5 years old, yet they are all highly rated. Personally, I've never had bad service from them and they fixed my frame problem back in 2009. I know Bud's Trailer in Burton is highly regarded by some friends of mine with SOB's, but getting into his place with a big 5'er may be an exercise. I do go there for some parts and accessories, though, since it's pretty close to me. Often, though, they have to order parts for you.
For "Grandma," if you are looking for a wide assortment of parts available off the shelf, try Richfield Trailer in Flint. They are pretty well stocked on all things trailer. I'm in Grand Blanc.