You have a good point. A holey Jeep will echange air quite fast at highway speeds.
A tub can be made to be quite air tight to maximize heat retention and the air can become stale. In fact I've worried a bit about Carbon Monoxide posioning and keep a little aircraft CO warning button on the dash.
However, if you think you've got enough air flow then seal up the fresh air vent grill and see what happens. What's it going to hurt. Just pull off the vent and put a piec of cardboard sealed with duct tape over the vent and rplace the vent grill. It only takes 10 minutes. Give it a try and let the Fourm know the results.
Before I sealed and repaired holes on my steel tub I could keep it warm down to about 15*f as long as the Jeep was at a stop light. The minute it took off from the stop light the air rushing in the rust holes was enough to change the volume of air in the cab at least twice and the temp would drop radically. But not before humidity started to build up.
Once the holes were fixed and the tub sealed up, the cabin would contain heat quite nicely, but then the humidity became a problem that was controlled by using the fresh air vent as a mixer. This solved the problem and I could still contain the heat.
In the old days lots of people would just crack a window 1/4" when they drove in the winter for the same purpose. This kept dryer air circulating and a fog free cabin.
If you do seal the jeep up, then hopefully there is positive pressure inside the Jeeps cabin and the heated air is going out side at a slower rate than the non heated air comes in.
With most Jeeps, it's just the opposite, that the outside air rushes in in such volume that it's almost impossible to heat, and capture the heat while at the same time postivily pressurizing the cabin... only with cold air.