Warranties are very profitable sales items which is why the salespeople are instructed to push them. If the cost of a repair is going to be more than the cost of the warranty and the repair is likely to be needed during the period of the warranty then it makes sense to pay for one.
When the DLP televisions first arrived they had lots of problems with the fans and lamps and master circuit boards and any one of these problems would result in a $300 to $800 repair bill. We bought an extended warranty and it quickly paid for itself. Now with LED L:CD TV' we don't bother.
The only items in an RV I would worry about are the inverter and the fridge. The other items are relatively reliable. If the RV has a 1-year warranty on the installed items I would expect that to uncover any defects.
One good thing about RV's is that the components used by one manufacturer are also used by many other RV manufacturers and the construction of them has changed very little over the past 20 years. It is going to be the new technology like tilting solar panels or similar innovations that are most likely to have teething problems where it is difficult to predict the reliability of all the parts that are used.
It is worth taking the time to review the owner's forums for a particular RV and see what problems they have experienced and the cost of any repairs. Best way to learn about any problem areas for a specific RV and its manufacturer.