What would be the advantages and drawbacks of a single-propeller tiltrotor?
Such an aircraft can't exist.

Helicopters don't work with a single rotor, they need at least two of them to counter torque. There are several popular designs, including conventional, coaxial, intermeshing and tandem rotors.

All tiltrotors operate as tandems in helicopter mode. It's the only setup where both rotors can be placed at the nose, tail or wings. In all other designs, at least one of the rotors needs to be mounted in the middle, above the center of gravity, where it can't be tilted without crashing it into something.