"Since I already have two breaker boxes, I would just split the new 50 amp cable, direct half of it to the existing transfer switch and the other half to the second breaker box."

Yes. (Each breaker box has a 30amp main breaker protecting the box, non?) (I didn't realize you already had two breaker boxes, but it doesn't change anything)

"..if I was on generator power, I would have only half of the 50 amps available, i.e., the half that was wired to the transfer switch?

Yes. If you want everything available all the time, you'll need a NEW TRANSFER SWITCH, NEW 50AMP BREAKER BOX, NEW (#6 instead of #10 SIZE) WIRING ON ALL THIS, and your generator probably isn't large enough to carry it all.

"But if I was hooked up to 50 amp shore power the second a/c which I would wire into the second breaker box would be able to run at the same time that the first a/c was running." Yes.

"If I was hooked up to a 30 amp shore supply, which breaker box would be supplied?"
Only the box on the BLACK wire - the "original" box which is tied to the transfer switch and carries all the original outlets and appliances.

Thanks Gunner. We're camping at the KOA Little River Campground in the Great Smokey Mountains park this weekend. First time out for the Barth in almost 1 1/2 years. Everything works. Will tackle the wiring to 50 Amps this next week. Sounds like a simple and very inexpensive way to achieve what I want to. Thanks again.
Jim