I have a Trimetric in our "home" to monitor the batteries. (Off grid solar setup in a 5-er) They're pretty dang awesome for sorting out a system, and keeping tabs on it, but for one battery in an application that only sees intermittent use, probably overkill and expense that you don't really need. The cheapo alternative that many people use is a monitor meant for testing rc car motors I think. It handles loads to and from a single battery well, with almost as much information as the Trimetric. It's called a "Watt's up" meter. Gives volts, amps, and amp-hours, similar to Trimetric.
On the really cheap and easy side, I put one of led bar-style indicators in my camper to monitor the house battery. (Check my build thread for a picture.) It works, but it doesn't start coming off full until the battery is down to about 75%. I basically use it to make sure I'm staying above that all the time, since I don't like the battery to get much lower than that. On longer trips in the cold months, with the heater running more, my solar is only adequate on sunny days. If it clouds up some, the indicator will come down off full over the first few days. It seems to be pretty smart, and does a fairly good job of monitoring battery status once it comes off full, and will tell me if it catches back up, or if I need to run the truck a bit in the morning, or clean the panel. It's not just a voltage meter, as it has logic and seems to stay low even if the panel brings voltage up during the day, if it's not actually keeping up with demand.
If I did it again, I'd probably go with a Watts Up, or do what Colorado Outback posted above, but use the volt meter that has voltage on the top line and amperage on the bottom. (ebay) Resting battery voltage can give you a good idea of the battery's state. Amperage would be useful to tell you if the solar panel and charge controller are working properly. Amp-hours is useful for a telling you if your setup is adequate, or how much power you used vs. how much has been put back in, so you can get an idea if you're borderline, or really OK with your setup.
