It would still need the 1/4" per foot slope of any drain. Then, the tank would need to be low enough to accumulate some so it didn't have to run every time something what dumped in it. Where is the sewer outlet? Above the floor or below? Do you need a pump at all? Better to bury it, if gravity would work for the whole thing - much less to go wrong, and much less maintenance. A rear-discharge toilet is more expensive than a conventional one, too.