Pretty much exactly what I did. I funds my flying while driving tankers on a generous tax free salary. 3 months at sea to study for ATPLs in one's cabin with no distractions, and then go home and spend accumulated salary on flying. As a Deck Officer the academics will make the "harder" ATPL papers of Gen Nav and Meteorology much easier as you will have studied the same subjects to similar or greater depth previously, allowing you time to focus on areas you find more difficult.
I managed to complete the whole thing with no debt and after positions with two other operators am now in the hold pool waiting for a call from BA.