There are a couple things that you need to look at and make sure you're not exceeding:
Hitch rating: That is the total weight of everything you're pulling with the hitch.
Tongue weight: The weight coming down on the hitch. With a tow bar, that's probably basically zero.
GVWR: The total weight the towing vehicle can support on its own axles - this includes the weight of the tow vehicle, people in the vehicle, fuel, cargo, water, and tongue weight.
GCWVR: Total capacity of the towing vehicle including the weight of the vehicle, cargo, water, fuel, and whatever is attached to the trailer hitch.
You need to make sure that you're not exceeding any one of those. You might have an 8,000 lb rated hitch, but if your GCVWR is 20,000 and the rig itself weighs in at 18,000 that means you can only pull another 2,000 pounds.