Did some work on "THE TOY" New alternator(180),new belts, new flooring for new starter batteries, new gage, some new wiring, new tentioner, new battery disconnect switch. Problem: Gage will read a little over 11 volts, start the diesel engine, gage stays the same until you press the throttle, then the needle goes to 13.7 and stays there, However,when driving when you stop for a traffic signal and the engine idles down the needle drops to 11.7. What didn't get done correctly ?? Or is this normal?....................

Those voltages seem a bit low. How accurate is your gauge? I would check it with a digital VOM to be sure, before doing anything expensive.

Is this the voltage right at the batteries? Wish I knew more about those oil burners...

I had the same issue. I traced the voltmeter wiring to the buss under the drivers window. I disconnected this and reconncted to the ignition run side of the ignition switch and now I get real voltage from the alternator rather than through a number of cables, buss and circuit breakers. I don't have a true ammeter on my coach so it is only reading voltage, but it is better than the way they built it.