Following factors may help you decide on the quality of RV listing and Owner:
Number of reviews
Rating on reviews
How promptly does the owner respond to your questions.
If an RV listing does not have enough rating then you can still communicate with the RV owner to get answers to your questions.
While all above factors are helpful the most important step to alleviate your concerns may be to visit the RV or do a Facetime/video call to get a first hand feel of the RV.