I built a large cabinate for my gear plus a 2000litre tank and the water in the tank stays at around 7,5 deg without the need for additional heating.
I do have a 1000kw koi tank heater plus thermostat set at 7 deg in addition to that I have a 45watt boat style tube heater with a frost-stat which I keep at the moment set to 5 degs...
I made sure all connections were GEKA quick release and where it was needed ball valves fitted inline...so I can isolate most of the connections within the system.
Polystreen 2x 20mm sheets with double sided foiled bubble wrap seems to work, The whole frame is made of 2x2's....yes it cost a fair bit to build but it's
built and works.
I have also considered getting a bespoke tank made for the van which is double walled/insulated for me next van i.e next winter.
Cheers
Dave.
Cheers
Dave.