There are two questions implied:
1) Should I buy any extended warranty?
2) If the answer to #1 is yes, then is GS the one to buy?
An extended warranty is simply an insurance policy, so the basic question is premium cost vs covered risks. With any insurance, the notion is that many (most?) will NOT get back as much as they pay in, but a few will recover much, much more because they have very expensive repairs. Routine repair costs will rarely equal the annual premiums. So what is your risk of needing repairs that cost in excess of $750/year (plus deductible).
As for Good Sam, their chief feature is the ability to pay year by year rather than an upfront payment for 3-7 years. That makes them somewhat more expensive on an annual basis if you ignore the interest you might have received on the upfront payment in some other policy. You will see quite a few complaints that GS is pretty good at finding escape clauses to avoid payments, but it's hard to judge whether they are significantly worse than any other insurer. Too many people fail to read and understand the numerous things not covered in a service plan, so get shocked when a bill is not paid. However, if I were buying one today, I would probably choose another insurer, e.g. the Cornerstone plan or RV Shield extended service plans.