It shouldn't be too difficult. The following steps are general in nature; other rigs may be slightly different. The 30A service coming in, the heavy cable, will terminate in a junction box somewhere and connect to the (probably) 12ga. wires going to the main breaker panel inside. Somewhere very near there I would install a separate, new 50A/240V breaker panel (a small one will do) to hold the new 50A main breaker and a few smaller ones; one for the second a/c unit. Wire the new 50A cable into the existing junction box and connect ONE of the hot leads, the neutral, and the ground to the existing wires going to the main breaker panel. Again from the existing junction box, run a 3-wire, 12ga. (hot, neutral, and ground) cable from the OTHER hot lead, the neutral, and the ground, to the new breaker box, connecting the hot lead to the 50A breaker, the neutral to the neutral bus bar, and the ground to the ground bus bar. Now the fun begins. A new 3-wire, 12ga. cable must be routed from the new breaker box to the a/c unit. This could be easy or extremely difficult and I don't have any specifics on how to make it easy. Once that's done, remove the old hot wire from its' existing breaker in the existing breaker panel and seal it off with a wire nut and electrical tape, and attach a label to it explaining what it was originally for and why it's not used. Make reference to the new breaker box so someone can find it should it become necessary. Do the same with the hot wire at the a/c unit. Now double check all wiring, make sure all connections are proper and tight, measure for any shorts or opens with a meter, make sure all grounds are truly grounded. Once convinced everything is o.k., install the new, proper size breaker in the new breaker box, connect the hot lead to it, and give it the smoke test!
This will provide 50A service to the new breaker while retaining the original 30A service for the rest of the coach. After trying to trace out a route for the new wiring to the a/c unit and finding it too difficult, it may be easier to bite the bullet and upgrade the whole coach to full 50A service. This would also require a new 50A cable, new 4-wire (or two-3-wire) cable from the junction box to the breaker panel, a new 50A/240V breaker panel, and moving all the existing load wires from the old box to the breakers in the new one. At the same time, determine the load each load wire carries so the loads can balanced between the two 120V hot sides of the 50A service.
Disclaimer: I am not a licensed electrician. The above is intended only to show the methods I would employ to upgrade my own coach, and is in no way meant to be a recommendation or be used as step-by-step instructions. I recommend the use of a licensed electrician, but should you choose to do it yourself, it is entirely at your own risk.