Saturn's got around 470 million sorium.
This is my design:
Shell-2 class Fuel Harvester 6,700 tons 101 Crew 185 BP TCS 134 TH 150 EM 0
1119 km/s Armour 1-31 Shields 0-0 Sensors 1/1/0/0 Damage Control Rating 1 PPV 0
MSP 17 Max Repair 60 MSP
Fuel Harvester: 1 modules producing 24000 litres per annum
Standard Commercial Ion Engine (1) Power 150 Fuel Use 10% Signature 150 Armour 0 Exp 1%
Fuel Capacity 100,000 Litres Range 268. 6 billion km (2777 days at full power)
This design is classed as a Commercial Vessel for maintenance purposes
The orders I'm using are the conditional ones. I use If Fuel Tanks full, unload fuel at colony within 4 jumps, and if fuel under 20% move to sorium gas giant.
I think the problem is the conditional orders can't specify what colony I want it to unload at.
Oh, And I searched the forum, and see that its a pretty inefficient design, but it should at least work.