Old. And not so unique...
Sorry, but that seller needs to get a few facts right.
Toyota made it and they didn't make very many
Toyota made exactly zero campers. Toyota made 1-ton cab/chassis that they sold to motor home makers like Winnebago, etc. who made campers.
There are lots of better examples of Toyota-based campers on eBay all the time, not to mention CL. Personally, I wouldn't give $800 for this tired old truck.
other reason is that's it has a dually rear axel which makes the camper good off-road, less likely to get stuck in the snow and mud
All the Toyota 1-ton cab/chassis were duallys. I'll not even address the 'fact' that duallys are better off-road...
However, if you are going to get a Toy-based motor home, something this size would probably be a good idea. From what I've heard (not a fact per se), Toyota stopped selling the 1-ton chassis to motor home makers because they were overloading them with 'large' campers, and the rear end wouldn't hold up.
However, I personally would hold out for one of the ones that had a fiberglas camper shell, and avoid the problems that this wood framed camper is sure to have.
IMHO, of course. YMMV and all that.