Many new RV's offer washer/dryer prep as a standard feature or add-on option. I'm of the opinion that they are worth having - with a caveat.
MOST of the combination units have a what they call a "ventless dryer". It's based on a "condensing dryer" system. They heat up the air in the inner drum and the water comes off the clothes in that drum as steam. This passes through holes in the inner drum to the inside of the outer drum. The washer/dryer then runs cold water over the outside of the outer drum and the steam condenses on the inner surface. It's then drained. For this process you are using cold water during the drying process. Most folks you talk to will tell you that the clothes never get completely dry with condensing dryers. Some of them work better than others, but that's the consensus I generally get from folks using them.
There is a combination unit from Splendide that has a vented dryer, and there is one from Pinnacle that has a convertible system - it will do both vented and ventless. I strongly recommend a vented system.
I'm trying to keep a running list of the options available at
http://www.momentumgunner.com/rvappliances.htmlWe will be looking at the Pinnacle washer and vented dryer in the next couple of months.
As for why they are worth having - take a look at most laundromats. Sit and watch for a while. When you see a lady bring in a very nasty dog bed that has fleas coming off of it, it makes you re-think the whole thing.
Does that happen everywhere? I'm convinced it does. I've seen that scenario and others that are as bad often enough that I don't care to risk washing my families clothing after stuff like that.