....several things reduce incoming solar per surface unit/surface area: the 1st is the angle of incidence of the sun (time of day; time of year) relative to your collector aperture (your solar panel); and the second is filtration (clouds; curtains; glass treatment; etc)...
You can measure this by buying an incoming solar energy meter and mounting it in various locations inside your truck (on top of your small solar collector) and data-logging (the streaming data) where you park your truck (home and work)...IMO, it would be a dubious endeavor to mount a 2, 3, 4, 5 watt mini solar collector inside your vehicle as a trickle charger, unless you are moving your parked vehicle every hour on the hour, to maximize incoming solar energy at the collector! If you have a parasitic draw (and this is in all likelihood the case, if you have dead truck battery after only 1 or 2 weeks parked!) and can't find it, use a "battery minder" of some sort every day when parked.
Off topic: some trucks (and other automobile manufacturers' various models) use steel brake lines that are very susceptible to early corrosion; so knowng this, I've always manually coated all my brake lines "from factory new", every year. I'm happy to read that your later model GMC has factory coated brake lines! Good news.
