There is no standard wiring color code for RVs unless a manufacturer has one, and that isn't likely. You might be able to get that information from the manufacturer if they will share a wire diagram with you, but many won't. If you cant find a schematic for it with wire codes, about all that you can do is to go to the monitor panel and check the wire colors there. You can check the leads to see what is on them by using a meter. One wire to each tank should supply 12V-dc and one(probably the green) should also be ground. There will be one wire for each step in the level indicator, most likely the 3 white wires.
You should have two ends to the group of wires and so you can simply match colors and test it. If the 3 white are the indicator lights you can change them if they should turn on in the wrong order, which will happen if connected wrong.