The location for our W/D was in the middle of the rig (Milestone 370flmb) in a closet which left us with limited options. There was no place for a dryer vent, and if we went with a stackable unit, we would have lost the use of the closest for any clothes.
We ended up going with a combo ventless unit from GE. I was worried about how well the ventless part of the dryer would be but we have been pleasantly surprised. The clothes feel kind of damp when you first take them out, but by the time you sort them and put them away, they are perfectly dry. There are eight of us traveling in the RV full-time so the unit gets a lot of use! The install was "easy" since we didn't have to worry about drilling out a vent and I just used the hookups that were pre-fabbed into the spot. I put easy in quotes as there weren't a lot of steps involved, but trying to actually get this heavy unit into the RV and into place was a bit of a challenge. We used the furniture wrist straps which helped a lot. The drainpipe that is pre-installed should have a diaphragm structure in it, so make sure you push the outlet hose down in there as far as it will go, otherwise, the water may not drain properly. I do not believe a drain pan would fit in with the unit even if we wanted one.