solar panels tend to work better cold, as they increase internal resistance with rising temps.
the "black glass" type are the least efficient. a 5W would probably be the bare minimum to make a difference. It's small enough that it wouldn't need a regulator, but would need a blocking diode to ensure the battery didn't drain through it while dark.
another mod is to replace the existing blocking diode with a schottky diode, which has lower forward voltage drop, which will squeeze just a bit more juice out of the PV panel.
The 12" x 12" panels used for solar attic fans pack a good bit more power in an equivalent size.
NOTE--- solar panels also want to see non-visible light. if it's inside the vehicle, some light is blocked, and the panel will not perform as well. panels encased in glass use a low-lead glass, if it is a quality unit.
Consider spending on a panel for attic fans at lowes or HD. add a diode to that. THEN monitor your daily voltages for a week and determine if you need to add a charge controller (not expensive thru harbor freight).