We almost bought a pickup with a cap, and intended to use our old Honda 2000 as a gen, but instead got the van and a tri-fuel Yamaha 1000 watt gen from US Carb using propane only.
For the pickup shell, to ventilate possible gas fumes, we had intended to add a couple of these small Atwood SS clam vents on the sidewalls, with a layer of screening sandwiched in. Maybe someone makes a screened version?
On boats, these vents are typically installed on a 45 degree angle +/-, to minimize rain intrusion from above, and water spray intrusion from ahead. The larger versions often have one angled somewhat forward as an intake in a protected area, if a bilge area needs positive air flow. The resulting minimal water intrusion is not seen as an issue, given the benefit of evacuating gas fumes.