How do you keep condensation out of a rooftop tent once nights get cold?

That dampness is condensation from your own breath, and once nights get cold it gets way worse no matter how many vents you crack. Two people breathing all night put out a surprising amount of moisture, and it rises, hits the cold shell of the tent, and turns into water right where you're sleeping. That's why the head end is soaked, that's where the fabric is closest to the roof and gets coldest. Cracking the rear window helps a little but warm air barely moves on its own, you need actual airflow pushing the moist air out.

A battery fan genuinely does make a difference, it's not wishful thinking. You don't need something huge, just enough to move air across the tent. The trick is placement. Point it so it creates a flow from a low intake point up and out through a vent near the head end, since that's where the moisture is pooling. I run a small rechargeable one all night in mine and the difference between fan-on and fan-off nights is night and day. Wake up and the fabric is just cool instead of dripping. Other things that helped me. Sleep with a window cracked at the opposite end from your vents so air actually travels through instead of just swirling. Pull the mattress out and stand it on its side when you're not in the tent, a damp mattress sandwiched between fabric layers never dries. And if your pillow is getting wet, throw a towel under it and just accept you'll dry the towel at camp, or use one of the thin synthetic pillows instead of a plush one that holds moisture.

On the packing question, you don't need to leave the whole tent open for hours. What I do is pop the top, pull the mattress out to stand upright, wipe down any wet fabric with the towel, and leave it open while I make coffee and break down camp. Twenty to thirty minutes of sun and breeze on the mattress does most of the work. If I'm packing up damp beacuse of rain, I'll pop it open again that evening at home or at the next camp rather than letting it sit closed for days, because a tent stored wet is how you get mildew and that smell never leaves.

Kinda relieved to read this because I thought something was leaking on mine. I'm in a truck bed tent, so a slightly diferent setup, but same deal once it gets cold out. One thing I noticed is my dog sleeping in there with me made it way worse, so if anyone camps with a pet that coud be part of it too. Also started drying the whole thing over the clothesline when I get home instead of at camp, way less rushing in the morning.

HVAC tech here so moving air is literally my day job, and the fan advice is right on. You don't need much flow, mine is a cheap clip-on running on low and it's plenty, because a little steady movement beats a big fan cycling on and off. One thing I'd add is where the moisture comes from besides your breath. I used to toss my damp jacket and wet boots in the corner at night and then wonder why the walls were dripping. Now anything wet goes in a bag outside or under the awning, and the inside stayed way drier just from that change alone.

Not buying the 'head end soaks because the fabric is closest to the roof' logic. In my old clamshell it was always the foot end that dripped, same roof distance top to bottom. The wet spot follows wherever your breath and body heat end up, and mine pooled at my feet. I also found aiming the fan straight up through a vent did less than just blowing it across the underside of the shell at a slight angle, like you're wiping the ceiling with air. That's what finally stopped the drips.

The fan helps but I'd also look at what's under the mattress. I run a wedge style tent too and for two seasons I figured the damp was all coming from above. Turned out my self inflating pad was wicking moisture from the cold aluminum base and the top fabric stayed clammy no matter how much air I moved. Tossed a cheap closed cell foam pad between the base and the mattress and the whole thing changed. Base layer insulates, less condense forms underneath, and the mattress actually dries in that twenty minutes of open tent time. Might be worth checking if yours feels damp on the underside, not just the top.