If I didn't claim the interest paid on my camper in 2017, can I add it to the interest paid in 2018?

No, unfortunately you cannot. You can only deduct interest in the year that it is paid. You could go back and amend your 2017 return, but the interest probably isn't enough of a needle mover to be worth the trouble of amending. If it is worth it, here are the instructions for amending a 2017 return.

https://ttlc.intuit.com/replies/6603004