Voltage limiter? Ohm's law will tell you a voltage limiter is a resistor, either purchased as a resistor or just an undersized wire for its load, voltage drop sound familiar? That's the old-school way of doing it. Let's look at that first. So, resistors...Those do a good job of lowering voltage and there are very good quality, inexpensive resistors available (blower motor resistors come to mind) but they're terribly inefficient. They'll turn your hard-earned battery power into wasteful heat (but not enough to be useful) with no benefit except letting your furnace run when it otherwise wouldn't. You can do some research/testing and determine the current draw of your furnace and the actual maximum voltage it sees (like put the battery temp sensor in an ice bath and see how high the voltage AT THE FURNACE itself actually goes) then determine what exactly you need to do. If a resistor's wasteful nature doesn't bother you I'd price blower motor resistors at a parts store for a variety of very common (thus inexpensive and readily available) applications like American fullsize pickups (I'd bet 87-96 Ford F-series are among the cheapest and can attest to their robustness, having replaced 1 after 210,000 miles of constant use strictly on the medium-high setting). These are usually 3 or 4 resistors on a single circuit board with spade connectors. Connecting one lead to the constant input and then moving the other from High (no resistance) to medium-high or medium or low-medium or low (high resistance) could give you selectable control over the line voltage for under $20 I think. Hit a junkyard (or buy new at the parts store) and get the blower control switch and knob and you'd have a slick little adjustable voltage setup. Might take a few minutes with a calculator and volt-meter but I'm certain I could MacGuyver it up and bet anyone else could too! You probably have ~15 volts coming into the furnace at worst-case-scenario and need <14.5 volts (call it 14.0 to have a little wiggle room) and let's say (because I don't actually know) that your furnace pulls between 15-20 amps total (blower, control board, etc) you're looking for a ~20 watt load. You want some cushion in wattage here so you don't burn up the resistor but the load (resistance) needs to be fairly spot on which is why I think adjustability/selectability is the way to go, especially since your input voltage varies with temperature.
In that vein, why not just use a potentiometer with an inline voltmeter? If the gauge says you're at 14.49 and your furnace cuts out at 14.5 just turn the voltage down to 14.0 or if it says 12.8 and your furnace shuts off at 12.5 you could turn it up a little.
I personally have successfully used a solid-state no-load flasher to allow a low setting on some heated motorcycle grips by using the flasher as a pulse-width-modulator [pwm] (on...off...on...off...on...like a blinker, get it?) on the low setting and wired direct from battery to grips for the high-setting. This allows me to warm but not burn my hands depending on which gloves I wear without wasting my bike's limited electrical power with resistors. PWM controllers are available in a variety of capacities (current ratings) and may or may not work for your application...I don't know what the voltage is running within the Webasto but if it's a computer or thermostat the on/off/on probably won't work like it might with a blower or fuel pump.
That last little bit was getting KINDA techy, the PWM via flasher kinda bridges the gap of low-tech and new-school. If you're into technology though you could put an Arduino controller in the mix and have a fully automatic system that regulates your furnace input voltage to within a very narrow range constantly. You could dial back the techyness a bit with a transistor instead of a smart controller but that requires some discussion beyond what you're likely to want.
So, tell me what you want and I'll be happy to help. Low tech, medium, or high???
