The spell banishment says in part
If the target is native to the plane of existence you’re on, you banish the target to a harmless demiplane. While there, the target is incapacitated. The target remains there until the spell ends, at which point the target reappears in the space it left or in the nearest unoccupied space if that space is occupied.
Does the lung or stomach of a gargantuan monster (e.g., dragon turtle) count as an unoccupied space for the purpose of using banishment to transport myself into it?