Low mileage is nothing to take into account. I guess that a cam belt may be. I would like to know how many owners, and how long this owner had had it for sure, just in the event they found something expensive wrong with it and decided to sell it.
I would like service records, both engine and habitation.
You do not say if at a dealer or a private sale. I would, if you are not sure of anything within a MH take someone along with you who is. A damp meter would be a small priced item to buy and take along with you. Good looking interior is important of course, but get inside all lockers / cupboards and bathroom, with the damp test meter. Check how long it has been for sale, and remember 1999 is an old age pensioner in vehicle terms. I don't suppose that the figure being asked is that which the seller will actually get.
Vans are built to do high mileages over a short period.
At 20 years old don't concern yourself at all with the living areas until is has been on a set of ramps and had a comprehensive check underneath.
Chassis, suspension, braking system components can all be completely worn out at that age so you need to know the condition as any of them can be expensive/uneconomic to repair.
The van had done 78000, there is a load of bills for various repairs and the blurb says 1 owner but I would like to see the log book to confirm. I have had 2 motorhomes so have a good idea what to look for just wondered if there was anything specific to the Bessacarr that needs looking at.