most of the gas/lp trucks had the park brake handle, hard to miss it
some of their models (usually electrics) had what they call a seat brake, the whole seat assy was mounted on a special bracket bent in an L shape and it went down in front of the battery to a pivot joint that connected to a rod that ran to a motor brake unit.
Since you did say this is an S30A then it shouldn't have the seat brake.
So yeah, pull the floor plates so you can see down into that area behind each wheel and see if you can trace where the cables run to. If the handbrake lever was removed then it would be a sure bet they are ran and zip tied to something either under the dash or down in the frame area somewhere.