Consider having an independent third-party pre-purchase inspection (PPI) performed on your behalf.
PPIs are money well spent and the results (which are proprietary to you only) may lead to further negotiation should there be any issues you (or the seller) are not aware of.
RV Inspection Connection (dot com) is who we've used more than 4 times and we have extremely pleased each time with the scope & content of their PP inspection reports to include the documented report(s) & NRVIA ratings they provide you with for both the RV itself and individually for all of its components/equipment/appliances, electrical, water, sewage, interior/exterior finish, top to bottom, etc. etc. Their reports include well documented text, detailed photos, videos (if needed) and the ratings.
RV Inspection Connection is a portal that connects individuals (i.e. buyers) with NRVIA Certified Inspectors nationwide -- most of which are full-time RV'er's themselves -- who perform PPIs to supplement their income.
Very reasonable pricing and well worth it.
Once you contact RV Inspection Connection (free of charge), they will connect you with a number of their NRVIA certified inspectors in the area where the subject RV is located -- each of which will then contact you personally (by phone), provide you with their certifications, and resume of experience and then discuss their availability and rate. You choose the inspector that you want.
Note: For a PPI to be conducted, it will require that the trailer to be connected to both shore power and water at the time of the inspection (as well as having the fridge turned on 12-24 hrs in advance). The seller should not take exception to you having a PPI performed at your own expense and if they do, then that should be a "red flag".
Nothing goes unchecked!
Best of Luck.