How hard can this stuff be ??
A lot harder than that. Empty fuel tanks do not a space habitat make. You need insulation, vacuum integrity, an accessway, electrical power generation and distribution, water, oxygen, CO2 scrubbers, heat, lighting, food storage, sleeping quarters, things to do to make it worthwhile being up there (i.e., research equipment), attitude controls, computers, gyros, communications gear, telemetry, antennas, sun shields, and a host of other things I've forgotten.
The original idea for Skylab was a converted SIVB stage. They ended up using the basic frame, but had to add a lot more systems than initially thought. That made it the most complex vehicle in its time ever assembled and sent aloft.