The 15000 lb might be doable if the capacity of the Buddy Store + another fuel tank is considered:
Internal capacity "give": 1440 gal (5451 l) (corresponds to 3x 480 gal tanks )
External capacity "give": 1x Buddy Store 300 gal (1136 l) + 1x FuelTank 480 gal (1817 l) = 780 gal (2953 l)
Total fuel "give": 2220gal (8404 l), JP5 @ 0.81 kg/l = 6807kg (15007 lb)
EDIT: In case of the X-47B, 15000lb is exactly half of its useful load:
Empty weight: 14,000 lb (6,350 kg)
Max. takeoff weight: 44,567 lb (20,215 kg)
Useful Load: 30567 lb (13865 kg)
"...On Feb. 12, 2018, General Atomics revealed the partnership with Boeing in a press release that also named a half dozen other companies that it would be working with on its MQ-25 bid. Pratt & Whitney will supply its PW815 high bypass turbofan engine,..."
Now that we know what kind of engine is being used for the MQ-25, it becomes evident that it will have an MTOM that's very similar to the X-47B.
X-47B, 1x Pratt & Whitney F100-220U: 16000 lbf (71,17 kN)
MQ-25, 1x Pratt & Whitney PW815GA: 15680 lbf (69,75 kN)