While flat towing you eliminate the hitch weight portion of the equation. Thats the good part .
As said, by adding a 5000 lb rated class 3 hitch in place of the OEM 3500 lb hitch will be something to discuss with the rv manufacturer. There is good reason for them stopping at a max rating of 3500 lbs.
I don’t know what added stresses will arise on the rv frame and components but the manufacturer should be able to explain it.
Are you looking at a Sprinter or Ford Transit rv ?
It might help you to ask the guys on those specific owners forums, what they do.
