I two bathrooms upstairs recently the water levels in the bowls seem to be decreasing. When you flush the bowl fills up but over time it decreases to almost no water. One toilet was recently replaced. Also when one toilet is flushed the other bubbles, the sink and tub drain slowly. Could all these problems be a blocked vent? Your input is greatly appreciated.

Bubbling drains do indicate a blocked vent. The bowl drains when other fixture are used and drain, that creates a slight vacuum in the drain and pulls water from the toilet bowl. The best vent rodding is done from the roof if it is safe to be up there.