Once the car reaches 1/2 a tank the needle on the fuel gauge drops immediately to empty. The car actually cuts off and acts like it is empty but it really has a 1/2 tank of gas.

first thought is that the fuel gauge is 1/4 off and the tank is empty enough to allow the fuel to run from the pump pickup
refill when the gauge is 1/2 way between 1/2 and 1/4 and see if the problem persists

Depending on your system, either the float is stuck or the sending unit is bad in the tank, or your gauge is bad. Rarely is it the wiring in-between, but that is also a possibility.

from what you are describing you have two(2) different problems
1- the wacky gauge- its acting like a grounding issue- wires are crushed/smashed on tank connector
2- not filling - venting issue- vent line to top of tank pinched
3 solution- run tank down to 1/4 or less and DROP tank down to inspect wires and hoses- sounds like at some point in time the tank took a hit and wedged itself upwards against the underside,pinched the vent line so there is too much pressure in tank and also crushing the wires/plug causing the erratic readings(worse with less weight as the tank moves more)
if unsure of how to do this- you can get a manual such as a HAYES book at auto parts story- good luck

It could be a bad fuel gauge in the dash unit, but....
Usually it's "The bad".
The fuel level sending wipe arm and windings on the fuel sender wear and send erratic readings.The only cure is a replacement fuel gauge sender.
The check engine light should not be related to the erratic fuel readings.
"The good".
If you have over 200,000kms or 120,000 miles on your van you need a new fuel pump anyway,so have a complete assembly put in which will have a new fuel sender on it.

Did the mechanic use an OEM replacement unit or an after-market unit? I think the original unit has a damping circuit to prevent immediate response when accelerating/stopping/cornering; the needle reacts slowly to changes in fuel level. If the new unit shows an immediate transient response, but settles back down to the actual fuel level, I'd just live with it.

ZAS

It sounds like the fuel sender is bad in the fuel tank.

If the fuel gauge drops off when you are driving then its a sending unit problem that means you have to drop the tank and replace the float and sending unit. This is a common problem for alot of cars because of cheap gas. If you have to drop the tank check the fuel pump canster for dirt because of cheap gas. Replace the fuel pump strainer and fuel pump filter. If the car or truck has about 100,000 miles you might want to change the fuel pump at that time also just to cover your back. If you take it to a shop they will charge you at least $900 to start. But if you do it yourself it will cost you about $200 dollars maybe alittle more.

I believe chevrolet has a recall on this problem having to do with wiring repair at the tank.