Are the pictures attached pics of the actual trailer? If so, it looks, very superficially, like some work has been done. My recommendation is to get your seller to explain everything they have done to the trailer, providing pictures and receipts where possible. If the axles have been replaced in the last ten years, all appliances are working, subfloor is NOT rotten, especially in the rear, floors and upholstery have been replaced, and polishing has begun, then I would say $6500 is not a bad price.
If, on the other hand, the trailer is essentially a 40 yr old field find that has had new flooring hastily slapped down over a rotten subfloor, and the interior "prettied up" superficially, then it is a candidate for the moniker "polished turd." It is these trailers that people buy thinking they can head on out traveling the country, only to find that months of expensive repairs are needed before the thing is safe to travel in.
Why does the fresh water tank need to be replaced? Have you actually inspected the trailer?