The problem being a cargo plane usually wants a fast approach and takeoff to eliminate threats near the runway. I don’t know if a B2 (or flying wing in general) would be good at that.
The approach isn’t “slow” in terms of aircraft speed but rather its duration. Shallower equals more time in reach of adversary weapons.
The design requirements for a C-17 to operate from a rough field, such as that which was built at camp bastion in Helmand province, are considerably different than for an aerial refueling tanker.
The rough field tactical tanker role is as far as I know presently served by a variant of c130 modified for special forces missions.