Is PA "warning" Rvers. (license).

I would hazard a safe guess, somewhere else in the statute or compiled law that you extracted that tidbit from, it is explained that the rule applies to commercial or "for hire" operation of that example vehicle combination, for example a transport driver towing a new fifth somewhere.

there's a lot of similar language in Michigan's motor carrier code (commercial trucking), that does not apply to private citizens operating a vehicle for recreation purposes.