We have friends we often travel with that have a 2005 Montana (35') with a stack-able Kenmore washer & dryer. It was an option when they got the trailer. They are more efficient and faster than our combo unit. Problem is, 50 amps is needed to run both units at the same time and you have to be careful of other things in use. Our combo unit will allow us to operate it on 30 amps but we switch our water heater and fridge over to gas when using the combo unit on 30 amps.
http://www.irv2.com/photopost/showfull.php?photo=19889
Our 2003 Everest (38') has the combo cabinet (picture below) just forward of the axles so not much is transferred to the hitch weight. Our friends Montana has the stack able unit in a cabinet just inside the entrance door.