The Allison transmissions with hydraulic retarder were used in some high end class A motor homes ; Foretravel come to mind but there are probably others .
Cost over a regular trans and extra cooling requirements kept them rare.
When the retarder is in use the fluid heats up in a hurry .
All automatic transmissions have oil coolers , some oil to water , others oil to air , occasionally vehicles will have both .
I have a July 2013 Allison motor home trans brochure , and don't see the same " R " for retarder attached to any of series trans in the paper work ; where the Allison manual that came with my coach ( 99 ) lists the R series . Could be there wasn't enough demand in motor homes for Allison to keep making them.
Could not say if there is an R series still available in Firetrucks or Highway rigs.
EDIT: Found an older brochure July 2010 , for Truck R/V Allison TRV series that lists retarder function . These brochures are too large for me to attach here in the forums . When I have a chance I'll look for some info that's current.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.