What you have is NOT legal and really should be corrected. You CANNOT have general use lighting and receptacles on 30A circuits, even with #10 wire. You also do need a means of disconnect at a detached structure.
You can use this feed to power a 30A-120/240V sub-panel using a tied two-pole 30A breaker in the main panel. This is only true if there are two hot wires, one insulated white and a bare or green ground. You CANNOT do this with only a 3-wire feeder.
You can use a small 100A main breaker panel being fed with the 30A feeder. The 100A main will only serve as the means of disconnect and the 30A breaker will protect the feeder wire.
I would take a very close look at that interior panel. From the look of your outside 4-space "panel", it looks to be your main service panel, and contains your main breaker and your neutral-ground bond. It's a bit like a "rule of six" panel: it appears you can/must shut off both breakers independently, e.g. The A/C will still have power if you shut off the 100A.
As such, the interior panel is a subpanel. It doesn't need a main breaker since the main panel is attached to the building.
So I would take a very, very close look at the panel interior and look at Siemens' line of panels. What I'd be looking for is a 16-20 space subpanel that is the exact same physical form-factor as your panel internally. That is to say, all the bolt holes are in the same place, and you could unbolt the guts of the new panel and they would just bolt up into your existing panel chassis.
Because the hairy-scary thing about changing service panels isn't the guts, it's uncabling/unplumbing all the cable connections around the edges of the panel. It's fairly easy to pop the breakers off (you don't even need to remove the wires) and remove the feeders from their lugs. Tuck it all back so you can get to the main busbar assembly, then swap it with one with more spaces.
From there, I would move the A/C load into the RV's main panel. Then use the breaker formerly for A/C, and use that to feed the shop subpanel.
The main panel's breakers total 125A, a common electrical service. I would not allow them to total more unless you consult with the electric company about the service they are providing you, and whether it can handle more.
Best Answer
Since the garage panel is a subpanel and not a service entrance, you should be able to turn the entire subpanel's feeder off upstream -- I'd lock it out with a breaker-lock + padlock myself to avoid a shocking surprise from somebody else in the family.
With that done, running the gazebo circuit into the subpanel should not be an issue -- it's like running wires into a giant junction box in that you'll need to support and clamp them in the same way as you would a box. Once you have the wires in there with enough slack to make the connection -- simply screw the bare wire to the ground bar, the white wire to the neutral bar, and the black wire to the screw terminal on a spare 20A breaker, presuming you have one. You'll also have to take the front cover (dead front) off and put it back on, but that shouldn't be too hard.
Once everything is hooked up, put back together, and ready to go, simply take the lock off the feeder breaker and turn it back on.