Just purchased 3 yr old house. Septic system w/ 2 holding tanks. Tanks were pumped when purchased. House was vacant over 1 year. First floor toilet flushes only with plunger. 2nd floor toilet causes all other drains to gurgle when flushed. 2 roof vents appear to be clear.

"2 roof vents appear to be clear."
Well something's not clear and it sure sounds like the second floor bathroom vent. Are you revented in the attic so you can't get to the vent from the roof? A gurgle from a trap tells you it's attempting to vent through the trap. Check the vents again. Good luck, Tom