Keeping warm in the winter in your truck camper can be a tricky exercise. Insulation values for truck campers vary widely, both by manufacturer and age of the rig. Where you go is another story, too, since truck campers are more on the line of "go anywhere" rigs, it's not uncommon to find their owners tooling them around in the snow country.
The typical factory equipped heating system is a small RV furnace, equipped with battery consuming blower, often ducting heat around the rig, and sometimes down into the holding tank area. In below freezing areas, this is an almost essential part of life--keeping the holding tanks above the freeze point avoids damage and makes for an easier journey. We found this out with an older camper of ours one chilly November at the North Rim of the Grand Canyon. Water began backing up in the shower pan when the shower's "p trap" was frozen solid. No shower that morning!
But the furnace option is just that: An option. If you spend a lot of time away from utility hookups, boondocking out in the wilds, that furnace fan as mentioned, can chew up a lot of amp-hours. You might go to sleep one night in the cold country, and wake up the next morning even colder when you find your battery's been consumed by a hungry blower.
What options are there? We've often commented on "blue flame" style heaters, that take no electricity to operate and put nearly 100% of their heat into the rig. But blue flame heaters aren't without problems: Some RVers complain that a blue flame heater in a small area like a truck camper can cause eye irritation. An alternative that seems to give fewer problems is that of a catalytic heater or even a "brick" heater. Still, both of these heaters have to be carefully sized, as by their nature most have no thermostat. It's a matter of turning them on, and picking a setting like low, medium, or high.
Most camper owners will find that the smaller the heater, generally the better. We have a small "Mr. Heater," essentially a portable brick heater. Even set on the low setting, ours can "run us out" if the night cool isn't significantly below 50 degrees outside. We have sometimes found it workable to turn the heater on, set it low, and then adjust window openings up in the cabover bed area to moderate the amount of heat. Almost seems a bit wasteful, but the alternative is turning the thing off, then jumping out of bed to the morning chill, starting up the heater, then quickly trotting back to bed until the chill goes off.
In any event, should you chose to use any of these alternative heaters that aren't vented to the outdoors, carefully observe the manufacturer's instructions regarding fresh air being brought into the rig. Yes, they do consume oxygen, and having a window open a crack may seem wasteful too, but keeping you alive is obviously a much more important consideration.