We have something like the smaller one. 10-12 gallons. Used it a couple of times when in a bind. Our camper doesn't have a locker big enough to stow it flat, so we had it strapped on underneath the chassis out of sight using 2" ratchet straps.
We wouldn't wait until the tanks were full before "decanting". If we were at 3/4 full and knew it'd be another couple of days before we got to a campsite or to home, we'd break out the tote. Not a pleasant job. But its a means to an end.
How to avoid over filling the tote? Ours isn't fancy and doesn't have a level gauge. In our case someone watches the LED display inside in the camper while another controls the decanting. Once it gets down to 1/4 full, we shut the valve. simples. We never FILLED the tote just to avoid any unpleasantness.
As for weight, I can't recall exactly as it's been years, but a half full tote was very manageable, at a guess ~20kg. Easy enough to lift onto most campsite waste traps and tip out the contents.
It would be much easier (and less gross) if campsites had ground level traps for RV's but I guess the demand isn't there. I think Jamie said he has a ground level trap for rv's up in Doonbeg, so you'll just have to go there a lot more!!
Maybe a future version of the Motorhome Parking Map might include a list of ground level RV dump stations!