Ah, the joys of a manual tranny. Really only have to worry about clutch wear, and not much else.
I believe in flushing regularly. ATF doesn't like heat, and will break down. Bad ATF doesn't do real well. If you do a lot of heavy towing, especially long distances, look into a manual torque converter lockup switch. This allows you to lock up the torque converter, so that it won't self unlock when reaching a gentle grade. An unlocked torque converter produces a LOT of heat. Sure, some people don't like them, but if used correctly, they can cut down a lot of heat and allow you to tow in OD.
Also, when pulling through town, I tow in 3. This usually keeps from the constant 3 - 4 shift/torque converter locking/unlocking, which cuts down on heat. This will kill your mileage on the highway, but is beneficial at stop and go.
Or, just get you a 5 speed and have no worries.