WOW that truck looks GREAT! Good job!
I've seen photos of M105 trailers with 46" tires (aka 395/85R20 or 15.5x20 as you have), there doesn't seem to be much clearance between the tire and the fender. Also some members here have run 14.50x20 tires from M35A3 on M105s, which seem to fit without too much trouble. But those tires are getting scarce today, unless you find some of the new MRAP equivalents (like 335s or something like that).
Simp5782 would probably know a close equivalent, and might even have some....
BUT, one easy option to share wheels/tires (with M35-style adapter plates for trailer, unless you swap out that axle & hubs) would be to use 2"x3" or 2"x4" rectangular steel tubing as spacers between M105 body & frame, a body lift for trailer, to provide necessary clearance. Heck, the trailer might even match up with the truck at that point.
I have plans (someday
) to do this to one or two of mine, so I can run 395s on them.