Your painter's opinion is the only one that counts. He has to guarantee his work and what is he willing to accept when you drop off your RV.
Unfortunately, I've heard painters that prefer different extremes, from do nothing to prime.
I'd personally be concerned with oil and exhaust grime getting into pinholes, scratches, etc. If it was my RV, I would probably prime, then let the painter remove the primer if necessary. My opinion is only worth what you paid for it.
I painted my cowl first, so it's a non-issue on my RV.
bob
