Q1 best warranty? A savings account full of cash. 2nd best warranty YOU! rvs really should be owned by handy people if not its a really expensive hobby.
Don't get picky about issues if you buy new they will hold the rig forever without fixing things (after you take delivery, ONLY be very picky prior to delivery!!). extended warranties have so many exclusions if your patient and stay on them and love the stress of dealing with them then you might collect 1-10% of what you paid.
Q2 best wheel and tire. Again YOU. A harbor freight jack and some powertools is all you need. This may or may not always be feasible so AAA is a good option. Some insurances also have these attached to their plans. Check your date codes and LOAD range on the tires all manufacturers use the cheapest rubber money can buy. 5-7 years from the code change them rubber rots from the inside out. Look at a TPMS system, avoid extensions on the stems check your air pressure regularly, look at single piece long valve stems such as tireman or borg dually valve.
Q3 AAA, some insurances also provide this free. If you maintain and inspect regularly all 3 of these questions can be avoided somewhat.
Really for the money saved from not purchasing any of these you will have more then enough to cover any problems that will come up along with the tools to do the work. AAA, insurance freebies are cheap enough to consider. With all the youtubes available anyone can fix anything.