Brakes locked on Holiday Rambler

Will they release using the caging bolt?

Drums? Mine have before. Freightliner. I put it in reverse and mashed the pedal.

It's the break. Pads pressed against the drums.

I assume you have adequate air in the system, and the parking brakes are off right?

moved technology

The best "preventive" is to use them/warm them up immediately before you park.

Parking "cold" and even worse cold and damp is a receipt for "brake shoes will not release".

Your hammer and rocking gently are the proper actions. I don't think caging them will have any impact, as I don't think it a failure in the air cans, but just that the shoes are sticking/rusting to the drums.

Yes have required air pressure, have set and released parking brake several times.

Because of the info above, caging them will not help. If you are not willing to rock it back and forth more aggressively I would chock the wheels, release the brakes, and hit the drums with a large hammer in as many places as you can. If this does not do it then I would take a long chisel and wedge it under metal end of the brake pad at the S cam that applies the brakes with them released. A normal pry bar might just fit and if all else fails a small sharp chisel into the end of the lining at the drum surface will but you will mess up a little of the pad but not enough to hurt the braking.

Your parking brake is on the rear axle only.

When you apply and release the parking brake do you hear the air exhausting?? If not then could be tiny diaphragm in emergency quick release valve has stock.

Have a much better chance of breaking them loose by hitting the accelerator pedal while in reverse. Not "drive". Tough to do but always worked for me.