Both my OFF ROAD TRUCK and JEEP Wrangler back in the day always had one sitting on the dash for keeping the start battery topped off... These were neat back then as they just plugged into the 12V lighter socket. Today it is very hard to find a 12VDC lighter socket that stays 'ON' after the ignition key is turned off.
I really should get back to this keeping my Truck start batteries and trailer batteries topped off...
Just keep in mind this only really works to top off your battery that is already charged up...
If your battery has been drained to 50% or lower than battery science tells you if you want to get it recharged in a quick three hour time frame than you need to charge it with 14.4VDC with a current capacity of at least 17A to 20A per battery in your bank.
Obviously the 7-10WATT small solar panels is NOT going to provide any of this...
Photos from GOOGLE IMAGES
One thing you should watch for is to make sure the SOLAR PANEL has the BLOCKING DIODE installed to prevent any loading to your batteries when the SUN is not out. This will prevent the SOLAR panel from discharging your battery.
You can easily use velcro to hold these panels in place on your truck dash.