NADA low retail price without any added options is a good place to start for the value. If it's in really good shape with new tires and has been well maintained, you might pay a little more than low retail. If not, maybe less. You didn't say if it is on a Ford chassis or a Workhorse chassis. That makes some difference, too. In my area, the low retail with 40,000 miles on the Ford is $29,414 and the Workhorse is $30,206, a slight premium to the Ford. It also makes a difference which part of the country you live in. RV's bring more or less in different regions of the country.
If the price is close to NADA for your region and it's in good shape and it's a floorplan you like then go for it. Be sure to check the date code on the tires to see how old they are. Looks can be deceiving. Tires more than 6 or 7 years old need to be replaced due to age regardless of mileage and that is a major expense these days. Also make sure you get a demonstration that everything works, not just the seller saying "everything works". Not that they would intentionally lie about it (although some do), but people forget about what works and what doesn't when selling an RV so try everything to see for yourself that everything works properly. It's always best if the seller has complete maintenance records so you can see that the maintenance has been done. If you're not familar with RV systems, you might consider paying someone to do an inspection of all the systems.
Good luck with your search.
Dan