Changed starter on the Van however it still won't start. Battery is charged and when you turn the key on it clicks once. Is there a relay switch or something between the battery/Key/starter?

Do you have a dvom you need too test for voltage drop across the starter + and - ciurcuit check for voltage from the + term. of the batt and the + post of the starter solonoid with the red lead on the batt. there shoould be no more than .2 volts while cranking do the same from the - post on the batt and the metal suface of the starter while cranking you will likely se a big jump in voltage thats resitance advom measures differance there should be none try cleaning the terminals and tighinig them check your connections at the starter and 12.6 is a charged batt less than 12.2 is not enough

You have a starter relay that could be bad, follow your + cable from the battery, it should lead to a small relay that looks like what is on a riding mower. If this gets cracked on the inside or the contacts are bad, this will prevent starting and a clicking sound will occur. If it's broke completely, it won't click.