So we have a plumbing issue that we just can't figure out.

When flushing our toilet, the toilet sucks the water out of the trap in the wash basin, but it does consistently flush ok and has not backed up. The other symptom however is that our bathtub drains very slowly - but doesn't appear to be clogged up with anything as it also drains.

Septic system was just recently pumped, it was getting full, but everything looked to be in order.

The only thing I can think of is that maybe the vent stack is clogged (haven't checked yet), but is that consistent with what we're seeing? Could something else be clogged?