Here is a final photo of the fence asphalt paint. It has hardened finally and is just what I was looking for. It isn't a smooth finish on the vertical areas, but I think this is likely just due to gravity and the nature of the paint. Even dry, it has a feeling of almost rubber.

Our 63 Shasta trailer has remains of a undercoating that looks similar to this and it's chassis was in much better shape for it then this chassis.

Again, it should not be used on the visible portions of the trailer, but for rustproofing the chassis...Perfect. I think it will protect for years to come.