Heating your van is one of those decisions where the "right" answer depends entirely on how and where you'll actually use it. Some people legitimately don't need heat. Others need something that works at -20°F. Let's figure out which category you're in.
For cold weather camping: Chinese diesel heater ($150-200). Cheap, effective, works in extreme cold, barely uses any battery power.
For moderate weather only: Skip the heater entirely. Good sleeping bag + van's built-in heater in the morning = free and simple.
If you have unlimited shore power access: Electric space heater. Otherwise, don't bother with electric heat - it kills your batteries too fast.
Avoid: Portable propane heaters (Mr. Buddy style). Carbon monoxide risk in a sealed van isn't worth it.
There are basically a few routes that you can go for heat in your van:
Honestly, if you aren't going to be ski camping or anything, I think for a lot of people you can get away without a heater just fine - once the sun is up, you can get a greenhouse effect anyway. Can get chilly at night but if you don't mind sleeping in a sleeping bag it's not bad.
If you tend to move in the morning, you have the vehicle's built in heater to use then too.
This is probably the right choice for people who mainly camp in moderate climates or only in summer/fall.
I need to call this out separately - personally, I wouldn't recommend this approach. These are the propane heaters you see at hardware stores that are designed for patios or temporary use. While they're cheap and require no installation, they come with significant risks.
If there is enough air flow, these shouldn't generate carbon monoxide, but they can if there isn't enough oxygen present - and in a sealed van, especially at night when you're sleeping, this is a real risk. There is also a high fire risk if anything falls near the heater, bedding touches it, or you accidentally knock it over.
Some people use them successfully, but the risk just doesn't seem worth it when there are better options available. If you're going to heat with propane, get a proper vented heater.
For most people building a van they'll use in cold weather, I think a diesel heater is the way to go. The Chinese diesel heaters (often called "Vevor heaters" or similar) have gotten pretty reliable, and for $150-200, you get a heater that will keep you warm in legitimately cold conditions without destroying your battery. Yes, the installation takes a bit of work, and yes, you might need to tinker with it a bit to get it dialed in - but once it's set up, it's hard to beat.
If you have a diesel vehicle, this is a no-brainer - just tap into your existing fuel tank. If you have a gasoline vehicle, you'll need to add a small diesel tank (a 5-gallon jerry can works fine and will last you weeks), but this is still manageable.
The name brand diesel heaters (Espar, Webasto) are great if you have the budget - they're more reliable, quieter, and have better customer support. But honestly, for a DIY build, the cheap Chinese versions work fine for most people.
If you have a gasoline vehicle and really don't want to deal with a second fuel type, then a vented propane system like a Propex is your next best bet. It'll cost more to run than diesel, but it's still way more practical than trying to heat electrically.
And if you're only camping in moderate weather, or you're tough and don't mind bundling up at night? Skip the heater entirely and save the money, space, and complexity. A good sleeping bag goes a long way.