If you own the newer version of Workhorse chassis, not the older Chev chassis, of that time period there is a ground bolt on your chassis
up front that has all the solenoids for your jack system white wires going to it.
The wire connections need to be clean and tight so the solenoids will open for jacks to work correctly.
Remove wires clean wire connection rings use di-electric grease and a star washer so nut and wires will not loosen.
The link also has info as to other ground locations plus your batteries need good cable connections to chassis frame.
You may also need new springs for jacks
to force the jack fluid back to the reservoir tank.
Some of us may carry a 2x4
to assist the jacks return in place.