Single Axle Leaf Spring
These systems are reasonably maintenance free. Eye-slipper springs are probably the most common single axle spring systems. They will wear the rear slipper guides and the front bush somewhere between 40-60,000km mark. The springs seem to flatten out at around 80,000km and the caravan can start to droop.
The other single axle system is the eye-to-eye springs which will also wear bushes at around 40-60,000km. If you have greaser pins keep them well packed with grease.