On an '87 345 there were two. One mounted between the tank and the air volume tank, and the other on the engine.
There was also a large filter/regulator just forward of the entrance door mounted on the frame.
Be aware there may be dry rot in the rubber lines, this would cause air to be pulled into the pump displacing the gas. Had a large problem with the Onan generator because of dry rot in the genny suction line.
Do several searches asking your question in different manners. Many GM chassis questions were discussed in the early to mid 2000's. All of the answers to questions are as good now as they were then.
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