I wouldn't switch to a diesel for pulling your trailer. The biggest reason for going to a diesel is more power not better economy. Not only will the diesel cost you more initially, but they cost twice as much to maintain as a gasser. However, if properly maintained, they will last twice as long as a gasser, so while not necessarily cheaper, you'll spend your money on things other than fuel. Now if you had a bigger trailer and needed twice the torque to tow her, then a diesel would be a no brainer.
The best way to save fuel: slow down and enjoy the scenery. Also avoid starting and stopping more than is necessary by anticipating traffic patterns as far in advance as possible. Take it easy on hills - let speed drop naturally going uphill and increase on the down hill as much as is practical while keeping safe, using your momentum to help you up the next. I like to travel at lower traffic times, (so you can do tricks like that) especially in the cool of evening - though you might miss some sightseeing if not traveling on a well known road. I don't recommend slipstreaming 18 wheelers though. That's one hypermiling technique that can get you (and some other innocent driver) killed. Gas prices are falling, but the price of funerals keeps going up!
Chip