Motor home with Ford F53 chassis

I have a 36 foot motor home on a 2000 Ford F53 chassis. (similar to single axel freight trucks and school busses) After getting one set of the rear wheels partially stuck, which resulted in some spinning of that side, the overdrive off light on the shift lever began to blink and the service engine soon light came on. The fault code said, “speed sensor showing more than one mile per hour”. Now the automatic transmission sometimes will not shift into first gear after stopping and has to be manually down shifted, after which, it works fine. Where is the speed sensor? Any advise or advise on where to look online would be appreciated.

Just so happens ‘Motorweek’ had a story on problems with ABS. First piece of advice was to make sure the ‘tone ring’ (the toothed ring that the ABS sensor reads) is clean, junk between the teeth might be causing the problem.