Are you asking about being able to use existing wiring of for a brand recommendation? If you want to use existing wiring, many of the camera manufacturers can also supply adapters to enable that. You'll need to look behind the monitor to see what kind of connectors your existing system has.
If you're asking about a brand recommendation, I didn't convert, but I recently added a system from Rear View Safety to our 5th wheel. I'm happy with the results.
PS: Rear View Safety does have cable adapters as does rvcams.com
. If you call with info on what your existing connectors are, they can probably supply adapters even if they're not shown on the website.