You do not need a hotel reservation to enter the Schengen area. You need to
justify the purpose and conditions of the intended stay, and  have sufficient means of subsistence, both for the duration of the intended stay and for the return to [your] country of origin or transit to a third country into which [you] are certain to be admitted, or [be] in a position to acquire such means lawfully[.]
(Source: Schengen Borders Code, Article 6(1)(c)).
So as long as the officers you speak to find your plan to travel and sleep in an RV credible, and that you satisfy the other conditions of entry, they will let you in.
A hotel reservation is only necessary if you plan to sleep in a hotel.