What do you know about him and his RV?
There are many small items that are useful, but he may have some of them.
A gift card, in my opinion, is a lousy gift.
While he can buy whatever he wants, a gift card also says "I put no thought or effort into this gift. I don't really care about knowing anything about you or putting any effort whatsoever into finding out what you may like."
You can just google RV Christmas gifts under $50 and you will get lots of hits.
But the key to any gift giving is to know the person, what they like, what they need that they might not necessarily buy for themselves.