OK. Only you've missed my central point. Towing as such, or how heavy your right foot gets, has no bearing. How did you think your fluid got black?
If you've done 14 dumps, do the maths, from memory, there's 8L total, and each dump drops 3L, so work it thru' and you'll see how much the old ATF is now diluted. A helluva lot. Should be pink.
Are you sure you've done 14?
14? Really?
14 x 3L = 42L
Unless yours is hte earlier 4-speed box, not needing it, 42L of JWS 3309/ Toyota iV has cost almost a new box anyway
About 15L will have it Dark plum, if black before. 18L shoud see it looking Dark Rose Pink as against fashion shades of light Rose out of the bottle. Magnefine it. If I recall right, 3/8" on 5 speed. 1/2" on 4 speed.