The only 5th wheels I have ever seen with a return so to speak of is the Open Ranges. This allowed the heat pumped into the underbelly to vent back into the utility area allowing underbelly circulation the vent was in the bathroom. I can say this works very well on my OR. I Full Timed in it 2 years ago thru 2 very cold winters with only a 600watt heater in the utility area that was wired to a 45 degree switch. It did not run much and was there for insurance. I had someone that came in for one week and had an Alpenlite 5th wheel and froze up good for 2 days.
Depending on where the Electrical Breaker box is the slotted step vent is for its cooling fans or venting heat from it. This what mine is for in the OR.
All combustion air is pulled in from outside. I have never seen a direct return line in and RV connected to the heater, only output lines.