There seems to be some confusion with a tandem breaker (two individual half-size breakers in a single unit width case) and a double-pole circuit breaker (two full-size circuit breakers connected by a single handle).
A tandem circuit breaker connects two independent 120V circuits, both necessarily on the same leg since it's a single slot in the load center. A dual pole breaker is usually used for a 240V circuit and on a residential load center would connect to both legs. In an RV, they're pretty much only used for the main breaker, and on many RV load centers in any other position it would only connect to one or the other leg.
A tandem breaker is fine in the situation the original poster describes.
