Since no one seems interested in designing something FOR you, I'll throw out my two cents:
You should concentrate on looking at CAR Radio Systems, many of which have a Bluetooth Receiver built-in, operate off 12-VDC and try to minimize battery drain. AVOID external High Power Amps if you want 12-hour Battery Life. And consider BOAT Radio Systems if you want it to survive some RAIN.
Also, you should run it off of a SEPARATE 12-VDC DEEP CYCLE Battery [or several Batteries connected in Parallel], just like an RV, with a separate Charging System and ISOLATION from the vehicle's Battery so you don't have to deal with TWO Dead Batteries at the end of your Gig. And like an RV, you probably want to PLUG into 120-VAC when you get back home to recharge your bank of Gig Batteries.
The simplest RV connection is to simply connect the Gig's Battery in Parallel with the Vehicle's Battery, so that the Vehicles's Charging System [hopefully] keeps BOTH of them charged. And there are Battery Cables available [for either Black or Red Cable] that include a simple Disconnect Switch so you can ensure that the vehicle will start when you want to go home. However, the vehicle's built-in Battery Charging Circuit will SHUT OFF when it detects that the COMBINATION of Paralleled Batteries has reached the target voltage....which may or may NOT fully charge the RV Battery, esp. given the extra voltage drop in the long cables....and the use of a DEEP CYCLE RV Battery. This arrangement is NOT a reliable system for your application, esp. since it will likely NOT be adequate to Re-Charge MULTIPLE Deep Cycle RV Batteries connected in Parallel to provide 12-hour of Gig Time.
You COULD completely isolate the Gig Battery Bank from the Vehicle's system and recharge each night using a 120-VAC Battery Charger, such are readily available for RV Systems (just don't connect to Vehicle's Battery).
A more complicated RV (or Boat) Battery Control System would ALWAYS protect the Vehicles's Battery from running down and would need to have sufficient CAPACITY to Re-Charge a BANK of RV/Boat Batteries [may require upgrading Vehicle's Generator].....but you'll have to do your own research.
Inexpensive Deep Cycle Batteries provide about 35 AH (Amp-Hour) of Power, which means the total Amp draw of your Gig System could be as much as not quite 3-Amps to provide 12-hours of playing time....although over time the Battery WILL degrade some, so good engineering would allow for up to 2-Amps Total in the Gig System [which is NOT very much]. There ARE Batteries that provide 100 AH....but they are very pricey....so you would probably need to buy multiples of the Inexpensive 35-AH Batteries.
You'll probably have to download the User Manual for each of the candidate Radio Systems before you will learn how many Amps each system draws....start out with systems that output no more than say 30 WPCx2, which MIGHT be adequate for a small group....or perhaps 100 WPCx2 for a larger crowd (and even MORE Batteries). There ARE a very FEW Systems that are TRULY "WATERPROOF"....and many that CLAIM it but are NOT (do your own research)....or the term only applies to the Speakers included in the bundle. Boat Owners & Jeep Owners running their Off-Road Rig thru the Car Wash are especially honest in their critique of the loosely used "Waterproof" term: