I occasionally travel for work and my truck ends up sitting for long periods. I'd like to be able to plug a cord into the 12V socket on the dash, put a solar panel on the dash and be reasonably confident that all will be nominal on my return. This seems like an excellent resource for vehicles that aren't daily drivers, but need to be able to start reliably when called upon....(BOV).

Anybody have one of those that meets or exceeds expectations?

Battery tender for plug in charger. Get the water proof one and install it under the hood. Solargizer for solar maintainer...just maintains it not charges it.

I bought a 10W unit probably 5 years back from tractorsupply.  If you get one, make sure it has a regulator.  10W will overcharge your batteries.  Mine has three modes to support different batteries including AGMs.  I've use to in my car during winter and also have carried on my boat to help keep the batteries up when I use the radio or fish finder but not run the engine.

RDK PRODUCTS
3W Semi-Flexible Monocrystalline Solar Panel
These Semi-Flexible Solar Panels are made of high-efficiency monocrystalline solar cells and are strong enough to be walked on

http://www.westmarine.com/flexible-solar-panels/rdk-products--3w-mono-semiflex-solar-panel-0-168a-5-88ah-dim-5-1-16-w-x-11-1-4-l-x-1-8-d--14996284

3 watts will maintain, but not overcharge your battery. No need for a charge controller.
They have  monocrystalline solar cells rather than the cheap thin film ones you find at harbor freight. It's much smaller and more powerful.
They are on a flexible plastic backing, and won't break like the glass on the thin film ones.
They are light enough so they aren't a hazard if you get in an accident and it goes flying at you.
The connectors that come with it are very high quality.
It has a led light to let you know when it's charging.

Will they work through a windshield?

Also, make sure your cigarette lighter has power even when the key is off.  A coworker left the country for two months and came back to a dead battery, and found-out the hard way his wasn't.z