I did both of the above suggestions, then bought an inexpensive digital "weather station" the just shows temp and humidity inside and outside. I bought it to have an outdoor thermometer, but found that the most useful thing was the indoor humidity reading. Once it gets upwards of 60% I know we don't have enough airflow to prevent condensation.
We use gizmos and the camper heater not turned up that much and a small electric heater and leave the door window opened about an inch. It was cold last time out and we were fine. The gizmos work great, won't leave home with out them
We usually crack the corner of the window on the feet end of the bunk and/or open the fantastic fan. That had always seemed to work. If we forget,we usually get condensation on those cold nights with the heat on.
2014 Rockwood Premier 2514G
2010 Ford F150 Lariat
Whether it was our pop-up or our new expandable. We always keep the tent end open just a little at the top. Just enough to let the air flow from one end to the other. Not enough to allow any rain to come in. Also the manufacture recommended we do this when we purchase our pop-up in 1992 and still works with our Palomino Solaire 199x expandable.
Vents help with the main body but they don't really help much with the tent ends. This way you are not buying any additional products.