We are currently constructing a large fuel tank farm. Once the construction of the tank is complete and there still is a reason to get on top for inspections of a low slope roof- is fall protection required if you remain 10' from the edge?

Yes, fall protection is necessary because the likelihood (risk) of breaking across that "line" whether 6 ft or 10 ft (and probably unmarked), is too high to ignore. The best solution may be a restraint system - but remember to watch out for the gable ends! Also, over time there will be corrosion of many tanks so that fall-through is a possibility - construct a fall protection system that is capable of going through the tank surface and will not cut.