It is certainly fine to shift in and out of tow haul on the fly. Sure it can drop to 2nd gear inappropriately, but tow haul can do that anyway. Tap the brakes on a downhill and watch the revs soar. You love 5000 RPM for no reason, you'll love tow haul mode!
I'm on the lighter end of E450 and personally only use it for downhill compression braking. It simply cannot read the road and be more useful than applying it manually when needed.
I'd much rather have paddle shifters.
I can see nothing about tow haul which would improve fuel economy-quite the opposite. For compression braking on as needed basis it is fine. For uphills if it selects a better gear for climbing it may be fine too. But to leave it on all the time....eewwww lol.