Tom,
There are three different problems that cause about 99% of the problems with sticking brake calipers on hydraulic brakes, and all three possibilities should be checked if you are having the problem.
First, and most common is slide resistance. Slide resistance can be caused from corrosion, bent or damaged mounts, worn mounts, or just a lack of cleaning and lubrication, especially if the coach is used on salted roads. All of these conditions should be checked when servicing the brakes.
The next most common problem is from a corroded or seized piston (pistons) in the caliper. If there is excess pressure needed to move the caliper piston, you need to either overhaul or replace the caliper.
The last reason for caliper lock-up is a faulty hydraulic brake hose. The inner rubber lining of the hose can come loose and act like a check valve to hold pressure in the caliper. This is usually an expensive repair because it is so commonly overlooked, and everything else in the brake system that was not needed is replaced first. Hydraulic hose failures become more common as they age beyond ten years, and since your coach is looking at 16 years, check those hoses carefully.
Gerald