If this is something that happened suddenly.. first thing I would check is the throttle cable at the engine end.
There is a red plastic sleeve inside the housing at the IP end. This guide sleeve breaks and then "stacks" the broken pieces so the cable doesn't pull the lever that open the fuel feed at the IP. It "feels" like you have the foot feed all the way to the floor, but you won't make full RPM... just like what you are describing..
To check, set the brakes... make sure you are in neutral, open the hood, start the engine, reach in and by hand, move the fuel lever to speed up the engine. By hand, you should be able to go to full speed on the tach.. [helps if you have some one who can watch tach while you do this..
If it revs by hand above what you can do with the foot feed... it's likely not your IP.. you will need to replace the throttle cable.. about 15 min and about $25...
Just a suggestion.. for what it's worth..
Amend that time line.. just re-read the original post.. it's a van.. add another 45 minutes ...