Next, while the engine was warm, I opened up the front and rear heater
valves and turned the fans up to high. The Temp went from about 1/2 to
almost 1/4.>>

John,

Two more simple things to do before you make any final decisions on
what's wrong  with the cooling system.

Take the Van for a high speed ride ( leave both heater valves on ) and
get stuck in a traffic jam.

If the radiator fan coming on doesn't drop the temp pretty quick, turn
the heater fans both on.

If the temp drop fast you have a clogged radiator.

Now to further develop on what's going on, beg, borrow, steal a infared
temp gauge, and with both upper and lower grills off hit the coils with
it in several different places all over when it's good and hot from
driving it on the road.

I know you did the palm test already, and found no cool spots, but I'll
almost bet you have an area in that radiator that the coolant isn't
flowing properly, and giving you hotter than normal temperature
readings.

One last thought.

My Van was doing the same exact things you are mentioning, and my
infared temp gauge told me the whole story right now.

A new radiator cured the problem.

The Van never gets above the half way mark.

Your's is just a classic, and common case of poor circulation.

Good Luck,