Allstate's ERS network is who Good Sam contracts with, so probably no real difference in the service provided. Prices and other details may vary, though. Just make sure anything added to your regular policy takes into account that it is a motorhome and not a car.
I think you need to be in the $10k range to get a decent full-body paint job. High end coaches spend more like $20k for their mirror-like finishes, but you can get a nice one for around $10-$12k. The difference is in prep work, number of coats, and between-coat sanding.
Some shops with good reps include Precision Painting and Mikes Custom Painting, both in Bremen, IN, and Bruce Deaton's Custom Paint and Body in Red Bay, AL. Mike's Collision in Wilmington, NC, is another shop that specializes in motorhomes and does a nice job.
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL