I installed one of these
Xantrex Linklite battery monitor, pretty pricey, but is small and compact for a truck camper and tells you everything you need to know. Instead of telling you volts it actually tells you what specific percent your batteries are at and tells you how many amp hours you are using and charging at the same time. If you plan on winter camping I would reccommend 1 more 100watt solar panel. Our 200 watt panel and batteries do not keep up in the winter and so we bring the generator along.