Was the 12.5 ton capacity based on the 26,000 pound vertical load on 5th wheel spec. of the M1088?
Yes, that is correct. I've never tested it and I doubt I ever will. For me, it is more an interesting bullet point to put on the sign when I display the truck at events. The most I have ever had in the back is around 3,000 lbs which the truck didn't even notice.
The hardest this truck has worked since I got it was recovering a halftrack that had been parked in the woods for 50 years. Once I got around the trees and hooked up to the halftrack, I was able to drag it to the street without any issues.
I did load 15,000 lbs into my M1090 dump truck when I had it and while the truck sat a bit lower than normal, it moved it without a problem. This includes going up an extremely steep hill that slows most commercial trucks down to 25 mph or so. The loaded M1090 climbed the hill without a problem at 45.