I think both arguments about the heating are lacking much merit. We have a vented combo, live in the Texas heat, and have not noticed any heating problems. Works great for us.
The vented models work just like a home unit and blow heated air thru the dryer and out the vent. So, yes, they do displace some of the internal air.
The unvented models take longer to dry clothes and use a lot more water. They put cold water into the machine to condense the water vapor from the heated wash load and drain it out the drain pipe. They do not put moisture back into the RV as some have said in the past.