In my old 2001 Monaco diesel I saw 9. Did not matter if there was nothing behind me, a trailer full of golf carts, a car on the trailer, or a mini van on a dolly. The only time I saw under 9 was when I ran the genset in the summer.
A friend has a Newmar Baystar gasser. They generally see about 6.5 without their subaru 4 down behind it, and just shy of 6 pulling the car along.