National Carrier may know within a few hundred pounds what most all their trailers weigh empty. Maybe give them a call and ask, see what they say.
Some of these reefers can weigh 16,000+ lbs.
My guess though, for the load they are asking you to haul, these trailers must be decent shape, and not too heavy.
Their company video boasts nice tractors, and looked like TK reefer units. I wish they gave more detail on the trailers.
If your tractor comes in 20,000 lbs. I'm sure you will be fine.
Don't know what size fuel tanks you have, but if you are mounting an APU, you will want a smaller tank by 50 gals. on the APU side to help trim weight.
For the loads they want you to pull, you probably won't get to run wet.
You'll most likely have to trim weight with your fuel, and listen for the golfball to rattle in your tank.
Put your driver on a diet, too.
Give him a company jacket 2 sizes too small.