I use two compassass in this Motor Home but first some history.
I've tried many magnetic (Compensated) compasses in my cars over the years and all of them have been less than spectacular, hard to compensate (Adjust) and I have never been able to get them to be properly calibrated for the entire 360 degree circle.
Some years ago I purchased an electronic compass, I think it was 49.95 or something like that the brand is Vector or if memory serves... Push a button , drive in a 1/2 circle and it's self calibrated. And as it turns out it was too, and every road I drove down it reflected what the map maker had written, very very close to true.
I then purchased a Garmin E-Trex GPS receiver So long as you are moving it can double as a compass
It too matches the cheap electronic job
I use the Garmin in the motor home as well, but also added another cheap electronic (Not the same make as the one in the car) performance is very simular to the one in the car (That is dead on) agreement with the garmin is also limited to the resoultion of the display (THe garmin is 1 degree resolution, the cheap electronic jobs are either 2 or 5)
So I'd recommend either 1: Both, or 2: A GPS receiver (See note below) or 3: An electronic compass
NOTE:
I also have the optional Data Cable for my Garmin, which allows me to connect it to my laptop, which runs assorted maping programs such as Streets And Trips (MS) or Street Finder (Rand McNally) and these programs not only plot my exact location (Well, usually exact) using the GPS data or provide direction.. I did note some interesting errors, back in Feburary on one trip, but nothing a reasonable person can not compeseat for.
NOTE TWO, there are a couple PCMCIA-GPS receiver/Software packages out there, much cheaper than buying independent Delor???? is one of the more popular one.. I actually wish I'd purchased that package