We bought a policy through RVProtection.net and have been satisified (only one claim, though). The policy we bought is underwritten by American Guardian, a fairly big name in extended warranty coverages. Most servicers will work with extended waranty insurers, who generally give a credit card authorization over the phone to the shop once they agree to the repair. Smaller shops may not want to spend the time/effort to call the insurer and discuss the problem and repairs, but those shops will decline any and all insurers, not just particular companies.
It is important that you understand that those companies are merely brokers who sell extended warranty insurance - you do not deal with them for repairs and it makes no difference if the agency goes out of business the day after you buy the policy. Before buying a policy, find out who the actual insurer will be and check out their reputation, not the broker's.
I have not heard of any complaints on policies from any of those three agents except the usual cases of people who did not understand the terms of their policy or did not precisely foillow the claim instructions. Carefully read what will and will not be paid for and the procedure for making a claim. You must do exactly as he procedure says or your claim will be denied - no slack at all is given by any of the extended waranty underwriters.
As Ned says, you want an exclusionary policy - one that covers all repairs except those specifically excluded in the policy. That cuts down on the misunderstandungs about what is or is not covered.
You need to decide how much coverage you want. We chose a high deductible policy with a lower price tag since we were looking primarly for coverage on big ticket items (transmission, fridge, etc). I handle most smaller repairs myself, so seldom need routine dealer repairs. If you are one who wants/needs to take your rig to a service center for routine repairs you may want a lower deductible, but be prepared for a high price tag.
Consider putting the price of the extended warranty in a bank account and using the money to pay for repairs. Odds are quite good that you will come out ahead financially.