It does seem strange you're overheating that badly. For that size of MH, even though it's small, you might be about at the limits of the transmission.
Where are the oil coolers mounted? If they're up in front of the radiator, I'd move them or install a couple more back in the frame rails. Install the types that have their own fans.
HD oil cooler with fanBack in the long frame rails I'd also install at least one of these on each side.
Oil heat sinksIt sounds like the transmission needs all of the help it can get. I'd also change to synthetic oil for the extreme temps you're getting - if you haven't done so already.
I'd also go to a larger oil pan such as the Mag-Hytec. They help cooling, but also the additional oil volume in the additional oil coolers and the oil pan will hold down the high temps longer.
Mag-Hytec oil pansBill