Check the fuel tank cap seals. On that airplane they have
O-rings that are supposed to be checked yearly (there's an AD on it) to
prevent leaks, both water in and air out. The O-rings get old, hard,
and cracked. A leak on the right side will create a slightly lower
pressure that on the left, and the left tank will drain faster. A big
enough leak will suck a lot of fuel out of the tank and collapse the
bladder.
Cessna sells a kit to replace those caps with the style we see
on the 172, minimizing the problem.
We run three Cessna 172s, an R182 and two Citabrias, all
with the same venting system, and as long as we keep the seals tight
the fuel drains evenly. As soon as a cap develops a leak the uneven
drainage shows up.
The vent under the left wing bothers some people, as they
think the left tank gets more pressure than the right, but if things
are otherwise sealed the pressures are equal in both tanks (Boyle's
Law) and uneven drainage is more likely due to restrictions in the fuel
lines. When the tanks are full or near full there's some fuel movement
between tanks (left to right) through the vent interconnect, but
certainly not 3/4 of a tank's worth.

The fuel cap AD:
http://www.airweb.faa.gov/Regulatory...C?OpenDocument

AOPA's take on the 182 uneven fuel feeding:
http://www.cessna.org/store/buyers_g..._excerpts.html
AN exceprt from that excellent article: "This condition can be
minimized somewhat by adjusting the position of the fuel vent behind
the lift strut on the left wing, making sure that fuel caps seal
tightly so that the "head pressure" in one tank is not altered by a
leaking cap..."

Dan