I have two 30 amp breakers (on separate poles) in my panel with 10-2 wire running to a junction box in my garage. The previous owner was apparently going to use these circuits for some RV application (at least the panel shows them as "RV"). Is there any reason I can't connect the two 30A breakers so the break together and use the wires for a 220v 30amp outlet?

It would work although the electrical code does require the circuit breakers to either have a listed handle tie or be a two pole breaker...
(I'm not sure about the handle tie, they used to be allowed but I can't find it in the 2011 code)

Either get a cap that locks them together or drill the handle and put a small screw through the holes so they both trip together. To be code compliant and real safe, get a regular 2 pole breaker.

Thanks. I did pick up a manufacturer identified handle tie, but it turns out the electrician (its a pretty new house- 2008) had actually already tied the breakers together but had mislabled it as the central vac. The two I'm tying together are already providing 220v power to the vac without having been tied.

A follow up question if I may. There is also a junction box just outside my garage door that has 10-3 wire back to two 20A breakers on a single pole. That box already has a single 12-2 wire from a third 20A breaker. It's labeled as if they intended to do a 220v/20A circuit (and I wouldn't mind using it that way) but I have always understood that the 220v needs to be coming off both poles. Is my understanding about the poles wrong?

Thanks. I did pick up a manufacturer identified handle tie, but it turns out the electrician (its a pretty new house- 2008) had actually already tied the breakers together but had mislabled it as the central vac. The two I'm tying together are already providing 220v power to the vac without having been tied.

A follow up question if I may. There is also a junction box just outside my garage door that has 10-3 wire back to two 20A breakers on a single pole. That box already has a single 12-2 wire from a third 20A breaker. It's labled as if they intended to do a 220v/20A circuit (and I wouldn't mind using it that way) but I have always understood that the 220v needs to be coming off of both poles. Is my understanding about the poles wrong?

220 does not exist anymore, it is 240VAC. Are you referring to 1 side of the box as a pole? Breakers are not taken from opposite sides. As a 10/3 you are referring to 2 hots, 1 ground and 1 neutral? You need 2 X 120 VAC supplies for 240. Straight 240 has 2 hots and a ground, no neutral.