good quality "ventless" dryers arent as bad as one would think...
the dryers recycle the air thru a heat exchanger that pulls the moisture out of it before sending it back thru the tumbling clothes... this moisture is collected and then is drained into the plumbing thru a drain hose that must be installed....
the washer dryer combo units are built to have the collected water pumped out thru the washer drain...........
the down side is, unlike a vented dryer that pushes the warm wet air to the outside, in hot weather, the ventless dryer creates more heat inside the RV.... the up side is, in the winter time, the dryer helps warm the interior of the RV, and there is no hole needed in the wall for a vent.
also, in the winter time, a vented dryer will pull the warm air from the trailer interior, heat it more, and then push it thru the clothes to the outside... this can take a LOT of the desirable heat out of the RV thru displacement... the proper ventless dryer CAN be a better way to go in most cases.