This is an interesting subject.
My old camper 97 lance 165 on a 1999 f 250 short bed I used a spacer made of 2x4 layed down with 1/2 ply wood cross pieces, it gave me about 2 1/4 inches with a rubber mat. Never gave it any thought, the guy who I bought the old camper used the spacer and gave it to me when i bought it.
I gave the Lance 165 to my grandson and gave him the spacer too, he has a 2002 Dodge, he says the heck with the space and goes without any thing between the bed of the truck and the camper. He has about 1 inch between the cab and camper. So far no problem.
My new to me camper 2002 Lance on my 2006 f350 I use 3 each 2x6 laying down and styrofoam between the 2x6s I seem to have plenty of clearance between the cab and camper eyeballing about 2 1/2 inches.
My question is how much room do you need between the camper and cab of truck roof, I don't know if 1 or 2 inches makes much difference in COG but if it did having minimum clearance would be ok for me.