I figure sle2115 has enough sense to use TWO manual valves.
I use WeatherHead myself and, although they are getting nigh impossible to find, I much prefer the ball to be stainless and not plastic.
The plastic balls are fine and will work for about three years, until the diesel-fuel finally swells the plastic to the point that they become so tight within the housing that the stem snaps off when trying to turn the valve.
I always buy the 4-port/3-tank valves, whether I need that 4th port or not; a simple pipe-plug will close it off and it is always there should one change things later.
As for the gauge wiring, it is easily accessed right at the old electric junk valve.
You should have THREE wires; one goes to the dash gauge; the other two go to the front and rear tank.
Put the gauge wire on the center terminal of a SPDT switch; put a tank wire on each of the end terminals.
Flip the switch to switch from one tank to the other.
As for gauging the auxilliary, I prefer mine being on it's generic SunPro gauge/sender, independent of the factory gauge.
If one prefers the auxilliary to also be on the factory dash gauge, then it is a simple matter to use one of the generic J.C.Whitney sending-units that matches the ohms signal of the gauge in question.
To switch the one gauge between three separate sending-units, a 3-position rotary switch comes to mind.