Our Dmax is a 2011. I have always used the exhaust brake on any significant downgrade, but not otherwise.
As far as the tow/haul setting, I alway use it when carrying the truck camper and towing the Airstream. I don't usually use the tow/haul with only the truck camper on board even though it is right at 75% of load capacity.
SuEllyn & Brian McCabe
WBCCI #3628 -- AIR #14872 -- TAC #FL-7
2015 FC 25' FB (Lucy) with HAHA
2005 Suburban 2500 Quadrasteer (Olivia) & 2011 Silverado 3500 (Fred) with Outfitter Truck Camper (Ethel)