Larry,
I never said a stock shuttle. But if you want 5 to 7 people and robust payload capacity up AND back, you start getting to a pretty large Trans lunar vehicle. You would need the crew compartment, a payload bay fully protected during aerobraking, heat radiators and the aero-braking structure which are at first blush a lot like wings.
Assuming that, you now have:

Shuttle-derived Translunar Crewed Vehicle.
Reusable
Aerobrakes back to earth orbit
5 person crew
Early missions would have fifth crewmember stay in lunar orbit

Thermal protection system would only need to handle heat load of deceleration
from 25,000 to 18,000 mph. It must be shown that a maneuver could be
made to skip back out to an orbit where storable propellant engines
(say new RS-72's) could circularize to a refurbishment station orbit.
Could the current shuttle tps handle such a load?

Possibly modify orbiter? (Thought exercise)
Replace 2 of the SSME's with 4 hypergolic RS-72 engines. For lunar insertion, decide on using a tank with a disposable engine that mates to the TLV, or keeping 1 (fully reusable) SSME and just attach a fuel tank. The latter would require demonstration of a capability to make leakproof cryogenic propellant connections rapidly by automated means in space. Add fuel tanks in back for the additional hypergolics. Add additional RCS fuel as replacement for front landing gear.

Delete rudder and landing gear, parachute. Body flap may be smaller.
Wings replaced by aerobraking structure of somewhat smaller size and mass.
Probable deletion of APU's - SPS was electric.

Your comment about 300,000 lbs of propellant is accepted for a stock orbiter. The actual vehicle would be limited in size to what could be sent up in 1 STS-C launch or 220,000 lbs or so. This gets us into the ballpark. But what else can we give up?
Would a cylidrical aerostructure with flap to protect the SSME save substantial weight?