They are available by special order at truck/trailer aftermarket parts retailers. The name Stemco in TFucili's post above sounds familiar to me. I think that was the brand name I had. Cost about $50, IIRC.
The counter sales rep may not know about them but if they're not too lazy they can be found in their catalogs.
The catalogs will list the hubometer by tire diameter or tire size, I've forgotten which.
I had one on my previous Travel Supreme but cannot take credit for the idea. I first learned they were available from a fellow TDR member I met at a TX rally or maybe it was at the Cummins MREP Rally in Columbus, IN summer of 2002 (date corrected on edit). He is a fulltimer and had one on his HitchHiker fiver.