Warranties are never necessary, they are more for peace of mind than anything else. RV warranties usually have so many outs for the warrantor that many issues that come up are routinely denied for one reason or another. They are huge profit makers for those who sell them.
That is a hefty price for an extended warranty, maybe you could negotiate a lower price making it an easier pill to swallow. Under the same circumstances I would bank the $4300 for future repairs with a good likelihood of coming out ahead in the long run. That sum would cover a whole lot of issues.
One thing that is paramount here is that the final walkthrough on this used RV, before taking final possession, is that all systems are displayed as fully functional by the dealer in question. Most systems on towable RVs are easy to diagnose and repair if one is handy with tools. Good luck!!!