If the van is insulated well, the D2 should be fine. It'll have to work harder, but it will, absolutely, keep you toasty. The D2 will work in this Sprinter, too, but I went with the D4 for several reasons.
The way the Airtronic works is, it'll go through its warm up, then fire up in BOOST mode for a few minutes (depending) to get things quickly heated up in there. Then, depending on the temperature of the air intake, it will cycle down through the HIGH, MEDIUM and LOW settings, and go up and down through those as needed to keep a relatively constant temperature. If the van is well insulated, the D4 will spend the vast majority of its time cycling between LOW and MEDIUM, while the D2 will cycle mostly between MEDIUM and HIGH. In sub-zeros, either will probably be in the HIGH range more than not.
In BOOST mode, the D2 outputs 7,500 BTU. The D4 outputs 13,650 BTU, nearly twice as much. While the D4 might be in BOOST mode for 3 or 4 minutes, the D2 is likely to have to remain there for 10-20 minutes before reaching the same cabin temperature.
BTU Heat Output
BOOST
D2 - 7,500 BTU
D4 - 13,650 BTU
In HIGH mode:
D2 - 6,150 BTU
D4 - 10,200 BTU
MEDIUM mode:
D2 - 4,100 BTU
D4 - 6,800 BTU
LOW mode:
D2 - 2,900 BTU
D4 - 3,400 BTU
Battery Power, AMPS drawn from the battery...
It's generally a good idea to use a separate, auxiliary battery for the Espar heater (and it doesn't have to be anything expensive or fancy, either), and anything else extra you may want to add later (like lights or an inverter). If it's really cold and you're stuck someplace for a day or more, all thw while running the heater, you really want the starter battery to be as fully charged as practical, so running the heater off an aux battery is a good idea. Be that as it may, the AMP draw off the battery is minimal, although relentless, and a 1 or 2 amp relentless draw can run a starter battery down over night, especially in sub-freezing temperatures. The amps drawn from the battery is largely to power the air intake and heat exhaust fans, and the electric start.
AMP Draw
START
D2 - 8.3 amps
D4 - 8.3 amps
BOOST
D2 - 2.8 amps
D4 - 3.3 amps
HIGH
D2 - 1.9 amps
D4 - 2.0 amps
MEDIUM
D2 - 1.0 amps
D4 - 1.1 amps
LOW
D2 - 0.7 amps
D4 - 0.6 amps
So, you can see the electrical requirements are, essentially, the same for both units. Of course, the D4 will spend most of its time cycling between LOW and MEDIUM (if very well insulated, mostly in the LOW range), and the D2 is likely to spend most of its time cycling between MEDIUM and HIGH (MEDIUM if very well insulated), so in that respect the D2 actually requires a little more juice over the course of a cold 8 hour night.
AIR FLOW, air intake, heated air output, in Cubic Feet per Minute (CFM)
BOOST
D2 - 48 cfm
D4 - 85 cfm
HIGH
D2 - 40 cfm
D4 - 69 cfm
MEDIUM
D2 - 27 cfm
D4 - 50 cfm
LOW
D2 - 19 cfm
D4 - 30 cfm
Weeeeel, that's not even close.
The D4 has a larger air intake and heat output hose, so considerably more air can be moved throughout the space with the same amount of amps of battery power. Which also means more BTU's can be moved in less time, more efficiently. On the surface, it's easy to think of air flow as nothing more than fan speed, but this is an important factor as it ties in directly with fuel consumption below. Less air flow means the heater need to manufacture more BTU's for longer periods of time, which affects fuel consumption, amps required, and at which level (HIGH, MEDIUM, LOW) the unit spends most of its time.
FUEL CONSUMPTION, Gallons per hour
BOOST
D2 - .07
D4 - .13
HIGH
D2 - .06
D4 - .10
MEDIUM
D2 - .04
D4 - .07
LOW
D2 - .03
D4 - .03
The D4 clearly uses more fuel at a given heat level, but because of the increased airflow and BTU output, the D4 spends much less time at the high levels, thereby using less fuel and amps over the long run. In other words, the D2 will do what you want, but it'll have to work harder to do it.
In my Sprinter, at temperatures between 0-20F, I find that it uses just about 1 gallon per 15 hours, or .06-.07 gallons per hour on the average. And I'll keep it warm enough in here to sleep in my birthday suit, often sleeping on top of the covers rather than under them. (If I'm gonna live in this thing, I'm gonna be comfortable.) The temperature thermostat thingy (I think it's actually called a rheostat, although an actual thermostat option is available) doesn't have numbers on it, but if they did, mine would be set at about 3 out of 10, and if it were a D2 it would be set at about 5. The D2 will use about the same amount of fuel, maybe slightly more, being that it doesn't have the BTU output or airflow to keep up, so it will run at the higher levels (MEDIUM and HIGH) for more time.
That's a small difference, when it comes right down to it, fuel-wise, anyway. But when you factor in the fuel, the amps, how much harder the D2 needs to work to keep up with a D4 in the same installation, the relatively small price difference between the D2 and the D4, for my money, was a no brainer. At least in an installation without a sealed bulkhead, like you are talking about. In a truck sleeper or a van with a bulkhead where you'd be heating only half the space, the D2 would be the way to go.
Incidentally, with either unit, if less heat than the LOW setting is required, the until goes into Standby Mode, and only the fan is running in LOW mode. Both units also have a fan-only mode that you can use year-round for general air circulation, which comes in handy in conjunction with something like a Fan-Tastic fan vent.
So, to answer your question again, yes the D2 will do just fine, but it'll just be running a little harder most of the time, is all.
