The ventless types are sealed units that, to dry, heat the clothes up to the point the water in the clothes turns to vapor and then is cooled and condensed by adding cool water to the unit to condense the vapor back to liquid water. All that liquid then goes out the drain, not into the RV. BUT. They take a LOT of extra water, longer to dry, and many feel that the clothes still come out damp feeling. Very few report being happy with them.
Separate or combo is a matter of opinion. Many prefer the separate units so they can wash one load while drying another. Others want the extra space for storage and prefer the combo. With the combo you can set it to wash AND dry, put in a load of laundry, go about your daily activities, and come back later to a clean dry set of laundry without having to wait around to move the clothes from the washer to the dryer. Whatever floats your boat.