There was a "Car Bar" years back that had a tank that sat in the trunk and it pumped drinks to a tap on the dash. Even had a section for hot drinks that warmed them up with an electric heater.
There's a similar thing for boats that I saw in Miami, but it was $400 and I can't remember the name. Could push one of 6 buttons and get 6 different drinks.
Or, buy a used soda fountain postmix setup that uses the tanks. Strap down a CO2 bottle and a bottle of soft drink syrup in the bed and have at it. I bought a setup that uses a hose-style dispenser with 8 buttons and a carbonator for $90 off Ebay. New O rings and some hose and it works great.