I had an older model falcon (EB GL Single point injection) that was quite ok for general driving across town but became asthmatic over 80kph. If I tried to overtake a car from 80kph it would kick down a gear and die in the Ar$e, Fairdinkum felt like a parachute was hung off the towball. It turned out to be exhaust related. the rear muffler had collapsed internally and was blocking gas flow badly....ok replaced that half....much better for a few weeks then started doing it again.....checked the cat...bingo another major blockage due to the guts crumbling. replaced that and it was sooo much better on fuel and went like a cut cat!(compared to before).
The jist of it was that short trips will root your exhaust much quicker than you'd think. The AU1 is an old car now but yours has only 100K on the clock. Average usage is about 20,000k per year. might be worth checking that out. mine was in a bad way but any amount of blockage will reduce performance and the extra load of towing will exagerate problems that you might normally not notice.
Cheers Roo.