Adam,
First, when do you notice the leak? If it is only after a fill-up or when the tank is full, it is probably the check valve in the end of the vent hose. This is usually accessible without dropping the tank as it will be in the end of the vent hose on top of the tank at the front edge of the tank and usually on the drivers side. On your coach it is normal to get a slight amount of fuel out of the vent hose if you try to squeeze all of the fuel that you can in the tank at a fill-up. However, if the leak is only when full and not from the vent hose, you may have a leak at the connection on top of the tank and this requires dropping the tank.
Next, if you notice the leak mostly when the engine is running, you may have a fuel return line leaking. However, if the leak is when the generator or heating unit are running, it is probably the return line for that unit, and both of these require dropping the tank.
The procedure for dropping the tank will vary depending on what chassis you have. The early production 98 Marquis (I think the first 26 coaches) has a Gillig chassis and the access to tank connections is usually a little easier than the latter production coaches that were on a Magnum chassis. If you have a Magnum chassis, you will be able to lower the tank enough to gain access to the top of the tank without removing it if the front of the coach is raised for clearance and blocked for safety with a minimum amount of fuel in the tank. However, you will not be able to remove the tank without removing the front axle, so if removal is needed, best see a good chassis shop.
Last, there are three other leak locations that will leak at all times that you have over a quarter of a tank of fuel, and they are the filler hoses that go into the side of the tank and the fuel gauge sending unit in the side of the tank. The repair method for these leaks is fairly obvious.
Gerald