On our '89 GV just now, returning from filling up the diesel fuel I couldn't get the shift lever to go into reverse. Had no problem at the fuel pumps. Just now. Pulled up hard on the lockout under the handle but nothing. Couldn't even get into Drive. Wiggle and percussion persuasion and cussing and was able to back the coach into our driveway, and then after a bit drive it forward up on to my ramps.
What's going on? Anybody got any ideas? Will have to wait until temps warm up outside a bit before doing any digging around to solve the problem. One other thing: Sometimes the parking brake light and the oil light would glow. I would turn them off by putting the shift lever in drive.
I am not sure if these are an "air shifter" or not. Maybe someone else can help with this. If it is an "air shifter" for your Allison: moisture in the air supply can freeze the shifter in gear. Carefully, follow all safety guidelines on this forum, you may have to get under RV and manually shift the gear selector on the side of the trans to move the frozen control.
On my '87 there was a small adjustment where the cable goes in to the transmission that I made when having the same problems.
My 88 is 100% cable. Might need the shifter lubed a tad.
Temp outside is 50+ and we drove about three miles each way to fill up and return home. Shift worked fine at the fuel station and I had idled the coach about 10 minutes after the block heater was on for three hours.
Seems mechanical, no air involved that I could see. Altho my little air compressor for the Hadleys runs quite a lot and often.
Might check the cable end down by the transmission and see if it needs some lube or rust buster like blaster.
Lift up the lid exposing the shifter and cycle the lever through the gears. Use WD-40 to something similar to free up anything sticking.