I had that problem when the batteries were in a low state of charge, I had it in for repair to the leveling system, and they had left the lights on, jump started the bus and brought it out to me with the steps extended, I had to sit in the car park and run the bus for 25 or 30 minutes before they would retract.
I then went and bought a set of jumper cables, so if I need to, I can jump the engine batteries off the house batteries, heavy duty cables 25' long.