The fuel gauge on my 01 caravan 2.4l reads full when running, and only reads 1/4 when off, any ideas what it could be?

You need to most likely drop the fuel tank out of the car and replace the fuel tank sending unit. Have this conformed by the dealer as it is a very laborious job and requires some special tools for the fuel lines.

Was the fuel sending unit replaced also?

Dont we wish it be full all the time but gas in it. Okay for the sending unit reading full its getting a full ground reading,Key on engine off. does it move and goe down. It should if it doesnt the problems with the gauge itself. 2 test disconnect the wire harness, did the gauge go down. the problem will be in the sending unit can be a hung float(Rare).

gas cap has no bearing on fuel guage. Sounds like float is broken in tank

fuel sending unit located on the fuel pump in gas tank

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Remove wire at fueltank end, if your gauge drops back to the bottom you have a faulty sender unit in your fuel tank, it is removed from where you just removed wire from, ensure your fuel is low before opening seal or you will waste fuel when you open it.
If your gauge still reads high after removing wire from fuel tank sensor, the fault lies with the wire itself earthing out somewhere along its length.
Have fun.

try lookin at the wiring harness under the battery tray,seen them corrode there alot

This is a normal concern.
The instrument cluster uses 4 different operating modes to calculate the fuel level:
Anti-slosh (default mode)
Key OFF fueling
Key ON fueling
Recovery
After a fuel fill up, the time for the fuel gauge to move from empty (E) to full (F) ranges from 2 seconds to 55 minutes depending on which operating mode the fuel gauge is in.
The default fuel gauge mode is called the anti-slosh mode. To prevent fuel gauge changes from fuel slosh (gauge instability due to changes in fuel sender readings caused by fuel moving around in the tank), the fuel gauge takes approximately 55 minutes to go from empty (E) to full (F).
The key OFF fueling mode (2 seconds to read empty [E] to full [F]) requires 3 conditions be met:
The key must be in the OFF position throughout the entire refueling of the vehicle.
At least 15% of the vehicle's fuel capacity must be added to the fuel tank.
The instrument cluster must receive a valid key ON fuel sender reading within 1 second of the key being put into the RUN position. The key ON sample readings are considered valid if the fuel sender reading is between 15 ohms ± 2 ohms and 160 ohms ± 4 ohms.
If these conditions are not met, the fuel gauge stays in the anti-slosh mode, which results in a slow to read full (F) event.
The key ON fueling mode (approximately 90 seconds to read empty [E] to full [F]) requires 3 conditions be met:
The transmission is in PARK (P).
The key is in the RUN position.
At least 15% of the vehicle's fuel capacity must be added to the fuel tank.
In key ON fueling mode, a 30-second timer activates after the transmission is put into the PARK (P) position (automatic transmissions). When the 30-second time has elapsed and at least 15% of the vehicle's fuel capacity has been added, the fuel gauge response time is 90 seconds to read from empty (E) to full (F). When the transmission is shifted out of PARK (P), the fuel gauge strategy reverts to the anti-slosh mode. The key on refueling mode prevents slow to read full events from happening if the customer refuels the vehicle with the key in the RUN position.
Recovery mode is incorporated into the instrument cluster strategy to recover from a missing fuel level input during a refueling event. Missing fuel level inputs result from intermittent opens in the fuel sender or its circuits. Recovery mode (empty [E] to full [F] approximately 20 minutes) is initiated when the following 2 conditions are met:
The instrument cluster is in the anti-slosh (default) mode.
The actual fuel level in the tank is greater than what is being displayed by the fuel gauge

check the grounds on your fuel tank sending unit,must drop the tank to do this. with the tank down,remove the ground and look at your fuel gauge it should read full or empty,if it does not it may be a sending unit

Sounds to be a faulty tank sender unit, these senders use a fine coiled wire on which a brush slides along to vary its resistance.
If the coiled wire breaks midway, the only readings will be full or empty.
I think sender unit is mounted to internal fuel pump on this model (top of fuel tank)
Hope this helps, regards