We have a GMC Dually with two extra leaf springs in the rear spring pack and a 5/8" block on the slide side. Our camper is a AF-990. I have checked and we are about 11'9" to the top of the A/C.
So a 13'0" door would work fine in you are concerned about overall height. I you want to hold the building height down. Use scissors trusses and go in the gable end, if you lay it out on paper you can have the ceiling just clear the door tracks and still have room to take off your camper inside if need be.
Good luck