The friendliest place on the web for anyone with an RV or an interest in RVing!
If you have answers, please help by responding to the unanswered posts.
The P0722 code means that the output speed sensor has no signal, i.e. the transmission controller can't tell the speed of the output shaft. As I understand it, it will not allow a shift into gear with that condition present. Unless you have above average skills, you probably need professional help.
What is the make, model, year of your RV and which specific Allison transmission do you have HD-747, 3000, 4000, etc? If it is an older one with air shift, you could crawl under, spray WD40 on the shifter arm and manually work it up and down and correct the problem.