So do you have wooden beam/rafters on the places where the two thinkness meet?
If the new material is a bit thinner than the old, one way to make them match on height is to add a furring strip on top of the framing. That strip needs to be the same as the difference in the two sections.
It often is not necessary to run the shim strip the full length/width of the new materail if the difference is small like 1/8 inch. Just let them taper down to avoid the full blown strip at all sides of the new material. Most people will not notice the taper!
And if they do, remember whose RV it is and invite them out but not to dinner---if you know what I mean!
__________________
Richard
Why no RV year, make and floorplan on MY signature as we suggest for others?
I currently DO NOT have one!