I think the main unit should have to be fueled. This way, when somebody leaves the game it becomes vulnerable and the space can be reclaimed. Then, a similar setup where there is a secondary, different fuel for the invulnerability timer. So if someone knows they can't get back to it within 48 hours, fill 'er up. But if someone knows there's at least someone every 6 hours then put 6 hour's worth in there. Or my favourite, 12 hours, so that it's unlikely the other team will be online when it comes out of invul.