Ford E350 air flow problems?

There may be a clogged cabin air filter some where .

Back then cabin air filters were almost nonexistent, so doubt it has one. In my old shops location we had a big problem with paper wasps build nests in vehicles, so everybody had a can of wasp spray in the vehicle. On your rig I would use a close mesh nylon screen under the grillwork as anything else would restrict the air flow.