I have lived off the grid for 25 years, and rely heavily on batteries, especially in the winter. Clean terminals are extremely important for optimal performance. When I clean the terminals, if I don't have terminal cleaner around, baking soda or even Coke works. Then as @LeonL suggests, sand/wire brush the contact locations to expose and roughen the bare metal. Once reconnected, I cover all the connection areas with a liberal coat of Vasoline. It will help keep corrosion from returning for several years.
Additionally, the batteries should be equalized every 3 months or so. This keeps deposits from building up on the fins of the core and improves the performance and extends the life of the batteries. My charging system equalizes the batteries automatically, but you could do it yourself with a regular battery charger. My system is 24 volt, and to equalize them I charge them up to about 30 volts and hold it there for several hours. The batteries will boil, which helps remove the deposits. Be sure to check the water level after equalizing, as the off-gassing during the equalizing period will reduce the water level.
It wouldn't hurt to change the battery cables either. Old ones can be deteriorated inside the jacket and you would never know by looking at them.
These batteries are 2 years old and have no corrosion yet, although they are about due for another coat of Vasoline.