I'd look at a toterhome with a real shower, dinette and bed. the Power and braking of a HDT would beat almost anything in the MDT lines
something like this on RJ is cheaper than a new F450 and has only 250K miles, you will never tax this truck pulling campers...
http://www.racingjunk.com/Toterhomes...Toterhome.html
you wont wear out anything on a HDT like you have experienced towing with a LDT this truck seems to be a bargin well below your 70K number @ $55K
good luck no matter what you decide