Dump zone pathfinding

I have several silos with assigned trucks to keep them empty. I don't really care where they dump the contents (so the new zoning thing probably wouldn't help) and I have several sites around the island.
The trucks have repeatedly picked a zone which has no road access (other side of a mountain I'm digging through) and have decided that they are going to sit there and do nothing until they can access that specific area, ignoring all the others.
Trucks assigned to a "keep empty" storage, if they select a zone like this, should automatically select a different zone that does have road access. They should only give the "having trouble finding..." error if they cannot find any zone to service.