Towing capacity

The ratings apply to all types of towing. Right up to the top of Davis Dam grade (Bullhead City, AZ) on a 110 F day.

I think egt's would also play a roll in towing that kind of weight.

thehippie wrote:
How about towing uphill? Will it still be 18,000 pounds?

Only if you make it to the top.

Towing yes flat road probably.
Thats a pretty heavy TT.
If your looking at a fiver towing really does not come into play. Load carrying capacity is your main concern.