Sounds like a cool little project, I'm tempted to say potentially you could get these items off the shelf from some cheap motor store, but where is the fun in that?
I have been experimenting recently and found one great thing with the RPi is the I2C interface on the GPIO pins, it allows you to connect a ton of devices (I think there are maybe 70 odd addresses) so you could get something like a BMP180/BME280 sensor which can do humidity/temperature/air pressure(at least I am pretty sure my BME280 does all 3), place it somewhere under the front bumper in a ventilated, but breathable box, so you could read all 3 devices, then have a little I2C OLED display in the cab somewhere in a 50mm gauge pod. You could alternatively use the DHT22 sensor to do humidity/temperature (its not I2C connection).
Basically its what I have been looking at as part of an indoor weather clock, to tell me what the inside temperature is and then outside have another sensor and display it on a little LCD. I actually have an indoor weather station that does all that, but I like doing things myself.
I think if I was designing the system I'd probably try to included too may features, but I think your going to need to look up:
Power - Raspberry Pi doesnt like being switched on and off like ignition, but you could make a little circuit to switch it on and off but run the circuit off the permanent live ignition.
Displaying on LCDs - I2C ones need less wiring (4 wires) as opposed to the normal LCDs that have 16 wires, if the PI is mounted in one place in your cab and the LCD somewhere else, thats lots of wires, 4 on the other hand aint so bad!
Using Temperature sensors - Start by looking of the DHT22, I have had mine displaying on an LCD with the time. Havent achieved using the I2C sensors to display yet myself.
Look up using the DS1306 Real Time Clock (I2C connection) so that your Pi keeps its time even though its out and about in a vehicle.
https://learn.adafruit.com/adding-a-rea ... pberry-pi/
What do you want to do with the compass you can get a little compass module too, found this on using them:
http://www.instructables.com/id/Interfa ... -Raspberr/
I guess it depends on what else you want it to do, feels a little more like an Arduino project, although I find raspberry Pi's easier to work with myself and I think there is certainly room for a media player in there too