The hitch weight is 8k lbs, but that's not the towing capacity. The GCVWR is the limit that both vehicles can weigh together. For my Ford E350 with an 8k lbs hitch, the towing capacity is still close to 5k lbs because of the GCVWR. I weigh my RV regularly on trips after filling with gas towards the beginning of a vacation. I have a really good idea of my RV capacity, weight distribution, and towing limits by using the scales to measure often.
My last trip (one week camping supplies, 3/4 fresh water) weighed 12,250 lbs. that drops my tow capacity to 6,250 lbs. At max RV weight (12,500 lbs), 6k lbs is the tow capacity. That includes the hitch and gas weight in the tow vehicle.
Just because it says 8,000 lbs does not mean you can tow 8,000 lbs.