93 Bounder won't move when tranny is in drive position

Hello. 1993 fleetwood bounder 34' motorhome, 44,000 miles, with a ford 460, and a 4L80E tranny. Driving on freeway slipped once, rpm's raised, thought it had downshifted climbing a hill, immediately released the throttle and reaccelerated and it ran fine for approx 100 miles. Exited freeway to fuel, came to a stop light, when it turned green I stepped on the gas and there was no forward movement, just raised RPM's. Went from"D" to "2" and limped it to the gas station. Fluid is normal in appearance and smell. Reverse, Neutral, 2nd and 1st work. Just not "D". What are your thoughts?