Somewhere, bolted to the frame of the RV, you should find a cylinder, probably 18" to 24" tall, 8" to 10" in diameter, with a bunch of hoses and wire coming to it. That would be your reservoir (tank), pump motor, etc. Up towards the top you should find a cap you can remove to check the fluid level. This is where you add the fluid as well.
On some rigs it is pretty difficult to add fluid because of tight quarters.