Problems with hatch size vs existing camper cut out size can be addressed by having a metal fab shop laser cut a stainless trim ring allowing new hatch to fit and trim ring to fit existing opening.
Another material option you could make/ cut yourself would be the plastic that is uv resistant called 'Seaboard'.
It requires double sealing bedding with the sealant however, first bedding the trim ring and then the hatch flange to trim ring. It's lots more work like this but solves a problem getting things to fit.
Sometimes I think 'RV' stands for 'Revamp'.
