Escape freshwater tanks are under the floor, between the frame rails, and behind the axle; in the 17' it's under the stepped-up part of the floor... except for the trailers of a very few people who requested custom mounting on top of the floor (such as under the rear bed in a 19').
Escape black tanks are on top of the floor, under the toilet - the toilet mounts right on top of it. For extra volume it usually extends into an adjacent cabinet; in a 17B (or 19') that means under the street-side front dinette seat. Early 17B models had a small black tank just under the toilet. That puts it well ahead of the axle on all current models, although just ahead of the axle on an older 17B.
Escape grey waste tanks are under the floor, between the frame rails, and generally just ahead of the axle(s) and behind the shower drain. In a current trailer that puts it behind the black tank and ahead of the fresh tank.