Having had a Superior on board tank on my 2004 senator, where the tank was easily filled and emptied from the outside, and held 40 liters.
The one on the unicorn is a joke.
In my option you are better off just using two 40 liter rolls, but this involves going outside to swap over, not ideal if its wet or winter.
Its also a problem to empty, you can try to pump it out, but that means you have to ensure the pickup tube is on the bottom of the tank.
Which means you have to remove cushions to easily access the tank, a real pain, not to mention the difficulty in getting to the change over lever.
What i have done is to fit a 12volt socket so i can fill the tank with a submersible pump from the roll. The tank can be turned from the outside of the caravan, and the pick up hose can be removed and refitted from the outside also,( you may need to remove a hose clip and move it further back, i replaced mine with a screw, as the nails are pretty useless, they pull out easily.)
This allows the tank to be filled. And the tank can be left in the reverse position.
I have found the tank difficult to remove with water in it, its slipped out of my hands, and on one occasion knocked the door off, luckily the clever design of the plastic hinge allowed me to replace it with out damage.
Another problem we have encountered is the pump going on air, after selecting the roll from the on board, via the hard to access lever.
Not sure why this happens, but the pump runs constantly, so quietly you can hardly hear it, i have found if i open the drain lever the pump will re prime.
I think there is still a requirement for a pump running light, it cant be good to have the pump running constantly. so make sure you turn the pump switch off when you vacate the caravan.
The old Vermont system had no levers but a separate pump inside the on board, all that was required was a flick of a switch to change from one pump or the other, i never realized how good it was until i bought the Valencia.
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