The 22.5's won't fit on the 20 inch rims.
You will have to buy a new set of Dayton AccuRide rims.
These are one piece rims and will run tubeless tires.
They will fit on the same spokes as the 20 inchers.
My bus presently has the 20 inch tube types with split rims.
I bought a set of 6 new AccuRide rims and am waiting for the dollars for new tires....
Here is a link to a page that shows the rims.
Mine takes part number F130391225.
The whole set cost 64.00 to have them shipped from north Texas to central Texas via truck freight and they cost about 68.00 each with taxes added.
Hope this helps!