There is no extended warranty except from the manufacturer. What is sold is an Extended Service Contract = ESC. It somewhat resembles a health insurance policy, describing what is covered, except with an ESC you want an inclusionary contract,which describes what IS covered, by omission everything else is not covered. An exclusionary contract specifies what is not covered, can you name all the other parts not named? Neither can I, which is why I bought an inclusionay contract.
Since this is a contract with binding terms and conditions, not strictly adhering to them may result is denied claims.
As to which is best, many RVer's choose an independent agency like wholesalewarranties.com
There are only a few guarantors who sell directly to the public.
For our circumstances purchasing an ESC was a good buy, some others choose to self-insure. Only your particular circumstances make the determination.
Personally I would not purchase one for a new travel trailer, but rather shop for one when the mfgrs warranty expired. The dealer is the last place I would choose to buy, the price is usually inflated and a money-maker for the dealership.