There are probably 100 discussions of exactly this question on these boards. About 10 seconds of searching will have you reading all day.
Every 30K miles you should have the transmission pan removed and the filter replaced. You should not have any kind of “flush” or fluid exchange without having the filter changed first. Any shop that wants to do this is looking to rack up $$ from their fancy, overpriced fluid exchange machinery.
Since you don’t know the history of this van you should probably have the pan/filter service done and then a full fluid exchange. If you wait until the fluid looks funny or smells funny or until the transmission feels funny it is already too late. This is like an alcoholic thinking s/he can keep sucking whiskey since s/he doesn’t have cirrhosis yet.
On that van the use of anything other than genuine ATF+4 transmission fluid will cause problems.