This image from the Star Wars Blueprints: The Ultimate Collection shows R2's drink serving arm, labeled #5.
However, it doesn't explain where the beverage reservoirs are located. The only fluids mentioned in the schematic are hydraulic fluid and lubricant, and I doubt that Jabba wanted R2 to poison his guests with either of those substances.
This is a modification so R2-D2 can serve drinks on Jabba's sail barge, but he's only serving them, not producing or dispensing them. Presumably they come from some bartender or a dispensing system of some kind.
As you can see, a customized tray has been fitted onto his hips and there's no obvious way the glasses could be dispensed from within his body, so dispensing the liquid too wouldn't be much of a timesaver.
However, some images, like this
and this Deluxe One-Sixth Scale Figure
do seem to clearly have tubes, which we can only assume means they are for dispensing liquid from within a reservoir in R2-D2. The image in OP does contain #14 Cargo Compartment as well as any number of other useless subsystems which could have been removed to make space for a reservoir (and cooling system?) as R2-D2 was no longer serving his primary function of Astromech.
This is not demonstrated directly, so we can really only say it's inconclusive.
