Stored inside, I just got 9 years out of a lightly used (less than 10k miles total) set of cheapo tandem trailer tires that hauled almost their max rating.
However I wouldn't have hit the open road with them for the last few years at least.
Trailer tires typically = replace them all every 5 years or so, or replace them 1 at a time on the side of the road through the new hole in you trailer fender for a few trips.
If they're not dry rotted, I'd fill em full of air, keep my speed down to 55-60mph and have a spare or 2 and a floor jack and head home with it.