Fuel tanks store fuel, and there is no civilian tanker functionality as yet.
I have some fuelharvesters with a crew MTHs at over 300, and its only going down by 10 a month, so no R&R for them. Thematically though I'm puting engines and 12 month deployment times on all of my ships except pdcs. Orbital habs are the only other thing that doesn't have engines, and I'm not going to build any megaformers/harvesters.