this probably isn't an approved method but I had a problem after I changed the fuel filter and couldn't get the air out of the system. I thought I needed to access the fuel pump from under bed (I was wrong, you access it from under passenger seat on a b series).
Anyway, while holding down the bed raise switch, turn the ignition over. The motor turning over will provide some power to the pump which will raise the bed if it's empty. You may have to do it in increments so you don't stress the starter. hope this helps.