I'm assuming your battery is 36 volt (18 cell). A new fully charged 36 volt battery will have about 40 V readings. When you drop below 36 Volts you are operating in an under voltage condition which causes higher amperage draw (heat) which will cause you problems. The simplest being burned contactor tips to big check book damage like drive , steering & hydraulic motor damage.

I would venture a guess that the top of the battery & on the sides of the battery case looks a lot like snow. That is an acid sulphate residue cause from the electrolyte (water & acid) which has bubbled out of the battery cells. The more white stuff you see the weaker the battery becomes. Touch it it will burn & make hold in your cotton clothing - jeans, shirts, etc

Solution: Get a new battery that is properly sized (weight & amp/hr rated capacity for that truck & for your charger & if your charger isn't big enough get one that is) or at least one that has been reconditioned by a reputable battery company. They should offer a warranty of some sort - if not walk away.
Either way the battery won't be inexpensive. That truck is 26 years old and is like a boat (a hole in the water you will keep pouring $$$ into). You may want to see want to look at a newer, pre-owned unit would run. There a finance to own programs for used lift to take out some of the sting.
Just my 2.5 cents worth.