Welcome to the forum. To answer your questions, almost all of the "value add-on packages" the dealership offers are a money maker for them and an added expense that seldom (if ever) has any value to the customer. The "spray fiberglass protector" is, in all probability a "sealant coat" that won't do much to prevent oxidation of the fiberglass after the first season. You can buy comparable sealants from EBay or from many auto paint retail stores for about 20% of the price charged by the dealer.
Often they don't apply them per manufacturer directions and don't prep the RV adequately to get a good finish. That just makes the product less reliable.
Extended warranties, fabric protection packages, paint sealant and window tint are all money makers for the dealership and their value is seldom (if ever) worth the investment.
As for slide hitches. With the new Cougar "rounded" front caps, there's little need for a slide hitch. It was designed to help increase backing angles on short bed pick-ups back when the fifth wheel nose caps were square. With the new rounded caps, almost everyone who has a slide hitch reports that they almost never, or never use it in the back position. The new front caps have negated the need for almost everyone. There are, however, a few owners who really like the "insurance" offered by a slide hitch and feel more secure with the option to use it if needed. Even they report that they almost never consider the need, "but it's nice to have"
The slide hitch is an extra $300 - $700 depending on which hitch you buy, so choose wisely and talk to other people who tow fifth wheels not just the dealer/salesman
Again, welcome to the forum.