In the August '05 Kiplinger's magazine on p.91 there was a question on medigap policy restrictions. The answer was:
"You're in luck. Most medicare-supplement policies can be used with any doctor or hospital that accepts medicare. As long as you get a traditional medigap policy - instead of one that offers discounts within a preferred-provider network - you should get the same coverage anywhere your RV takes you.
That's not the case with medicare HMOs, which generally limit coverage to certain doctors and hospitals, except in emergencies.
For prices of medigap policies, go to the Personal Plan Finder at
http://www.medicare.gov or your state insurance department's web site.
For a customized price list, order Weiss Ratings' Shopper's Guide to Medicare Supplement Insurance ($49; 800-289-9222;
http://www.weissratings.com).