May depends on specific model. You could try measuring the size of the tank and go from there. On my '07 300QB, grey is under the kitchen galley and black under the toilet in the water box (both port side), and fresh is under the bed.
Haven't figured mine either (have to fix a gray water piping leak first). I guess you can empty both grey and black water tanks and fill it with measured buckets until it stops draining.
Linear Mode
