Why does my 04 caravan drink a lot of gas?

You'll need to know how big the fuel tank is on your truck. They used different sized tanks depending on the box or body fitted. I think the standard body box was a 26 gallon tank and the long body box was a 34 gallon. This will help you work out your actual miles per gallon.
That model's factory figures have it at between 12 to 13 mpg average. How and where you drive your truck can also affect you fuel consumption quite a lot, City driving, Hard accelerating, Lots of steep hills where you drive and Driving Off Road all can ruin you MPG figures. Using the your numbers that does seem a little lower than it should be. When was it last given a full service? Have you changed fuel supplier, (different gas station). All of these affect MPG Do you have any warning lights on the Dashboard? Use the comment button below with more information and I'll try to help you more.

Using your numbers here's the math.
26 gallon tank - half tank = 13 gallons. 34 gallon tank - half tank = 17 gallons. You got 104 miles.
Sum used is 'Miles achieved divided by the number of gallons used'.
(26 tank) 104 divided by 13 = 8 Mpg.
(34 tank) 104 divided by 17 = 6 Mpg.

Fuel tank is leaking or a leak in the fuel line.

Could be bad regulator

It sounds like a leaking filler hose. Not uncommon, and not a big problem to fix, although getting the old cracked hose off and a new one installed can be a dirty and frustrating job. After investigating, you may find a previous owner had once pulled the tank off and just forgot to tighten up the filler hose clamps. , as if fantasy could ever come true.

worth checking the fuel tank for holes or there might be some fuel delivery pipes leaking badly. Or it might be an O2 sensor not giving correct signal to ecu thus consuming so much fuel.

Unless gas is leaking somewhere all the gas a car uses goes through the injectors. If it uses too much fuel that means the injecters are being kept open too long or it is being burned improperly or engine load is extremely high. Unfortunately it can be a pretty complicate process to figure out what the problem is on a computer controlled car. It taks some rather sophisticated equipment to monitor the engine sensors. Are there any codes stored in the ECM? Is the air filter clean?

Sounds like the sending unit is shot, which is in the gas tank.

it sounds like it could be a vapor lock in the gas tank leave the gas cap off for a while without driving it and check out the fuel filter while the gas cap is off and that why you won't lose all your gas

Probably easier to replace fuel tank sender unit as constant on off condition heard or not cant be good .Potential electrical spike.