Leonel, the first ship I sent to Jool with extra fuel for the return mission was essentially a nuclear powered craft with extra fuel mounted as a sizeable fuel tank attached at the front. It was a crazy single launch that used 64 of those long solid fuel boosters to get everything in orbit in one go. The plan was to get to Jool, do an aerobrake, get into orbit around Vall, discard the fuel tank, dock with the lander that needs rescuing, and then have a fully fuelled up return craft complete with lander. The absurdity of my plans then called for the landing on yet another moon before returning home.
Obviously what happened instead was that nothing worked as planned and I had to use up most of my fuel getting there and getting in orbit around Vall. It almost seemed as if my craft was so heavy that it took a lot more v to get it moving.. well, obviously that was the case, but it seemed to have a quite significant impact on fuel consumption.