this is great info, thanks Dave & Brian in particular for jumping in here. Keith I suspect you don't mind the hijack here, as this is directly relevant to your project, yes?
So say you were presented with one of the new, low cost, turn-key solar charging solutions for RV's. Something like this one, a 200w deal: http://www.ebay.com/itm/Complete-Kit...-/261297690217
It comes with a pair of solar panels, cheapo charge controller, some plug & play wiring, and some brackets. Presume that you wouldn't ever be using this to try to breathe live back into an over-taxed battery, but instead it's just used for camping trips... where an AGM-type battery measuring 12.4 volts when running the fridge gets a little positive drip from this panel.. end goal being at the end of the hot day, it doesn't go much below 12.1, and if it can start tomorrow at 12.4 even better. Best case is your truck could camp indefinitely with hot sun and cold beer, worst case is you can last another day or two, compared to no solar.
How much better would you think results from using a "better" charge controller would be compared to the cheap one? Not asking you to quantify it, just gut feel- worth another $100?