Welcome to the forums porknbeens, I have a 73 excella 500 so probably I can best answer your question. All the tanks are below the floor and are covered inside the belly pan(aluminium sheet covering the bottom of the trailer).
First the fresh water tank. It is located just in front of the axles. You will find an area that is boxed in with angle iron. The forward or front piece of angle iron is bolted to the side pieces of angle iron. Remove these bolts, most likely you will need to grind off these bolts as they will be rusted on. Once the bolts are gone the front piece of angle iron will come out. That will reveal the front edge of a 1" thick piece of plywood covered from below with an aluminium sheet of belly pan. That plywood is what the fresh water tank is sitting on. Now this plywood may still be sealed at the edges to the angle iron. You will need to break the plywood free from the angle iron. The plywood may also have swelled up due to water leakage or the angle iron has rusted for the same reason. The plywood will eventually be able to slide forward revealing the fresh water tank. Once the plywood is about halfway out support the tank so it won't drop to the ground. All the hose and pipe connections are accesible under the kitchen cabinet. Don't forget to disconnect the wiring for the levels monitor.
Second the waste tank(s). If your unit is a 73 it will have both grey and black tanks. Your profile says 72 so which is it?
The location of the waste tank(s) will depend on which location your bathroom is in.
Center bath units have the tanks just aft(behind) the axles. Grey first and then black behind that.
If your trailer is a rear bath then the tank(s) will be at the rear.
Either way they are located under the rear peice of belly pan. It will require removal of this belly pan for access.
The waste tanks are mounted in their own galvanized steel boxes which are then sitting in angle iron frames. Once again you will need to remove a section of the angle iron to drop the galvanized box and the tank.
Each tank has an imput water pipe connected to it and an air vent pipe as well as a drain connection. All this plumbing must be disconnected first.
For the black tank the toilet must be removed and then the flange on the floor is threaded into the tank.
The gray tank will have a pipe connected with a flexible connector.
The vent pipes are attached with the same rubber connector.
Both tanks will also have wiring attached for the level sensor system
Once you provide some more info as to the year and bath location and get into it I can answer more specific questions.
Why do you want to remove all the tanks anyway?