FIFTH WHEEL PROBLUM
I have a new truck and fifth wheel with a Reese 20K hitch and have the same problem. My dealer that I bought the RV from installed the hitch and I'm getting ready to take it back. I'm not happy with the installation at all. Have heard about this clunking problem with Reese hitches before yours and mine, so Reese might look into this before their reputation starts to suffer.
Buckbs
2002 F-350, PSD, dually, 4.10, auto, LS, with tranny, pyro, boost gauges
2002 Newmar 35ft. fifth wheel 35LKSA