Annie
Best thing since sliced bread!!!
Mine is 40 litre (a standard tank in an Abbey) so same size as the Aquaroll - that means 80 litres on tap when both filled up for the night. An electric switch operates the Aquaroll pump to fill the onboard tank, no valves to faff around with. The only time you would switch valves is to isolate the onboard tank to run off the Aquaroll only, such as for overnight stops, but that's a very simple process anyway - just turning a manual tap.
Yes, the onboard tank must be drained for travelling - 40 litres equals 40 kg in extra weight but that's done simply by another tap to drain out under the floor - no hassle, you should drain the hot water tank as well anyway. If you want a little water for the journey then use the Aquaroll.
If you are looking at buying a caravan with an onboard tank then check that it's factory fitted with all proper controls, anything missing and it won't operate as above - the control panel pump switch should switch between the pumps as well as 'off' (i.e. power from the taps goes to the correct water tank) and the inlet has an automatic non-return valve (to avoid the onboard pump pumping back into the Aquaroll). Apart from the drain, the only manual valve is to isolate the onboard tank in order to use the Aquaroll alone. Very simple and a very flexible use of water tanks - and access to 80 litres.
I wouldn't be without it!
Alan (another Alan!)