Dennis,

Were fluids drained or periodically changed?  Was it inside or out,
covered?  Tire maintenance?

Re spiders, at least inside the cab, you could put out some diatomaceous
earth, either directly on the floor or in shallow containers.  I would
not want to cover mechanical components with it; use at your own risk.
The idea is to get them to walk through it.  It's said to tear the
dickens out of their joints, which is fatal to them.  It's a little
cruel, but it beats getting bitten.  I would also put some roach traps
in it.  Spiders will tend to leave if the buffet gets sparse.  Having
dealt with an infestation, I fully believe there comes a point when the
spiders simply feed on each other, but even that can be reduced with
time, and a truck is probably not big enough to support it anyway.  I
took a house from the point of being unable to enter a room without
seeing at least two spiders walking around in the open (they were
everywhere) to normal levels (they were around, but you had to look for
them).  Along the way, the numbers dropped, and I started seeing bigger
spiders with time; I could be wrong, but I think the decreasing food
supply brought the bigger guys out in desperation, and then they died
off.  Good luck.

Bill