Well I just went looking for it. I though they were in Supp-7 but I don't see them there, only demountable tanks. MegaTraveller has them costing Cr472.5 per DTon (Cr35 per KL) and taking up no space while empty. They aren't in LBB2 or LBB5 and I don't see them in T20. Not in Supp-7 or 9. Of all places. OK here are the rules, paraphrased.
CT Collapsible Tanks. They take up space in the ship's main cargo hold. Empty they take up 1% of their full tonnage. Fuel must be pumped out of the collapsible tanke to the main tanks to be used. Cost Cr500 per ton.
Source: Adventure 5 Trillion Credit Squadron, Page 13.