i would run the battery through a couple of full charge cycles
if it does not recover enough to run the truck effectively then you will wind up having to replace the battery.
A good charged 36v battery should read in the neighborhood of around 37.5 to 39v depending on the age and condition.

Once they get so old and have been charged many cycles over the years they just wear out, batteries only have a pre determined number of cycles and that combined with the type of care or non-care it has received over the years they will eventually get to the point they will just not charge up anymore. Nothing you can do to fix that other than replace the cells or replace the battery.