Chuck
If your coach is air over hydraulic or hydraulic over hydraulic brakes the oem calipers were made by Dayton as was most of the suspension parts .
Dayton has since been acquired by the Bendix corporation the 55252 number is a Bendix number.
Those calipers should not be too bad to find as they where used by Ford on there medium duty trucks F700 and F800 in and around the mid to late ninety's.
They were also common on International 3800 series school bus chassis of that era.
There are companies out there that sell rebuilt units for as little as $100. USD .
Those calipers are set up to be used on right or left allowing one part number to do all four corners of your coach.
As you can see there are many companies selling the same caliper under their own part number as can be seen in the cross reference section of the link below.
I have used the Centix calipers I mentioned earlier and they work fine on all four corners.
Give your local truck parts supplier a call and give the either the Bendix number or the Centrix number and they should have no problem getting you what you need.
https://www.fleet-products.ca/55252-hydraulic-caliper-2-88-dayton-walther-ford-gmc-chev-e14684006/?srsltid=AfmBOopXJPh33j_yI6fj3A4qBUFRj0w85-tGW82gTfksbn5KF7kbkLEOEric