"Can" he claim it on Schedule C?  It sounds like he WANTS it to be a business.

As Camp pointed out, there is a good chance it would be on Schedule 1 instead of Schedule C.

HOWEVER, RV's are dangerous territory.  Assuming the RV has a place to sleep, cooking facilities and a toilet, then using it for personal for 14 days (or at least 10% of the rented days, if greater), would make it a "residence" for your client.  That essentially would mean many deductions would no longer able to be used.  So your client would likely want to use it LESS than 14 days a year, unless he wants to forfeit many of the deductions for renting it.

As of course any many of the allowable deductions (for example, vehicle insurance) will need to be split between personal use and rental use (probably based on days actually rented; not days available for rent).