My first TC had an aluminium 2" by 3" frame that was coated with Linex black truck box coating and raised the camper to the right height. I too found that the TC moved about, so in the slots I built just like you are designing, pieces that center the rig as it lowers and hold it secure there.
Instead of building a fancy front and back thing, I say build the frame that goes under the camper to lift it, spray it with the rubber Linex, and then put in the slots that will fit your framework.
Sure worked for me, and the framework went out with the old camper that needed the lift, the newer campers did not.