i have a 1999 ford econoline E250 van and I get a constant smell of gas while driving, Thank you in advance for any help.

 

If you can smell it, it is out there. The fumes are not god for you and there is always the danger of fuel leaking where it could get ignited.
If you cannot see any apparent leaks, with the engine running, you may need to remove the doghouse and make sure there are no leaks under there. Look for any hoses that may have come off and are allowing fumes to get into the vehicle. If all appears OK, then you are going to need to check under the vehicle where the fuel lines come from the tank and go to the engine compartment.The metal lines usually leak where they are clamped to the chassis or sheet metal because dirt and salt get in there and just destroy the lines. Look for wet spots or dark spots on the fuel lines. If the fuel filter was replaced recently there could be a leak where the lines connect to the filter. Keep in mind that those lines are under pressure all the time the engine is running. Depending on where you drive ice melting chemicals raise heck with steel fuel and brake lines and need to be inspected on a regular basis, monthly is not too often.