I can only provide my opinion on your inquiries. Others may have different opinions or experiences.
1. Extended Warranties: I wouldn’t purchase an extended warranty on any product. I try and buy the best quality I can find for my money and rely on the manufacturer’s warranty.
2. Paint and Fabric Protectant: I wouldn’t purchase either of these. You can buy aftermarket spray on fabric protectant at any auto supply store for far-far less if you want it.
The exterior siding isn’t listed as aluminum or fiberglass in your post. We have a 2005 Weekend Warrior with fiberglass siding and it has held up really well with simple wash & waxing. The most frequent cause of exterior appearance problems is either physical damage or peeling decals.
3. Slideout Room Cover: I have no experience or opinion on this subject.
4. Barker Power Tongue Jack: This is well worth while and I would at least attempt to require it as part of the deal, $300.00 shouldn’t break the deal from the dealer’s side and would be nice to have.
Good luck with your purchase and enjoy RV’ing.
Hap