Your batteries are done. A good set fully charged should be 25.2 volts or so. The "corrosion " inside the cells is actually sulfation. Wet cell batteries in an EPJ last about 3-4 years if maintained properly. And don't ever replace just one or two batteries. Always replace all 4 unless one fails within a short period of time after replacing them.

I'm not real familiar with Raymond's but most newer EPJs have a controller that needs to see a certain voltage in order to reset the BDI gauge. Your batteries are so low that you are not going to reset the indicator.

Added: I have seen batteries last longer but most of my customer base only gets 3-4 years. Grocery industry is by far the worst.