"i've only got three wires coming from inside my rv walls that go to the water heater . they are BLUE, ORANGE AND YELLOW. ---
my replacement wh has 4 wires that have been cut from the other rv wall of donor rv... ther are RED, BLUE , GREEEN AND ORANGE."
The wire color in your RV may or may not match the wire color in the donor RV. The colors are what the installers used.
The wire color in the two water heaters are shown in the wire diagrams that you can no longer understand. You must determine the function of the wires in your RV in order to connect them.
Do you know what color in your RV was connected to what color in your old water heater? If not, testing will be required.
Donor Green and Yellow
Look at the colors in the donor heater. Not necessarily the color of loose wire from the donor RV.
There are three 12 volt wires plus a green 12 volt ground in the donor heater. The green is 12 volt ground. Connect green on donor WH to ground. Yellow in the donor RV is also 12 volt ground.
Meter your RV wires to find ground. It should be zero volts and zero or very low ohms when measured to frame ground.
Donor Red
Red on the donor is the 12 volt positive and must be connected to the remote “GAS” panel switch.
Meter the leads from your RV. Switch the “GAS” switch “on” and “off”. Once found this identifies the wire to connect to the red wire in the donor WH.
Donor Blue
Blue is the DSL light on your control panel. Connect it to the remaining wire from your RV. You may wish to leave this disconnected for safety until you are sure everything else is working.
The "DSL" light on the control panel will not light if you left it disconnected. Connecting it to the wrong wire can cause smoke and fire in the donor water heater.
Donor Orange
Orange is 12 volt positive for 120 volt relay. Your old system does not have this switch. There is no wire from your RV to connect to the donor orange. It requires a new control panel switch like the 12 volt “GAS” switch.
The existing “Electric” switch is a 120 volt switch and must not be connected to the donor orange.
There will be at least two more wires from the old installation. They are 120 volts AC from the control panel “ELECTRIC” switch and a neutral wire. There may be an additional frame ground wire.
The 120 volt from the switch is usually black. The 120 volt neutral is usually white. Do not connect the 120 volt from the switch to the donor water heater.
What should you do?
If you have come this far, you may understand that new wiring and a new switch are required. Should you continue, or should you hire the work done?
At this point, the gas functions should be operational. The 120 volt electric is not going to work until wiring is modified and a new 12 volt electric switch is installed.
Hopefully Old-Biscuit will review my instructions and note any errors.
I wish you good luck and happy trails ahead!