Forward fuel tank

Most (but not all) boats have a valve in your fuel line for selcting the tank you use, a shut off valve near the tank and/or a valve between them to transfer fuel. They're pretty obvious. If you haven't found it I suspect you don't and it operates on gravity or each motor feeds off a different tank (a system I don't like). When full that tank is about 1,400 lbs. Having it empty could affect balast, and leave you riding bow up, stern down. That could increase your fuel burn, slow your getting on plane, lower your speed and make your motors work harder. 200 gallons is probably only about 5 or 6 hours of running time. If I remember the song, Catalina is "26 miles across the sea" or about 2 hours each way give or take. Assuming your aft tank is of similar size that gives you total running time at 10 to 12 hours. I wouldn't mess around.

Correct although some prefer to come down equally, but you don't dump that capacity. Definitely play with it and see how she rides best. Also, many places give a volume discount on fuel and that extra 200 gallons may get you there.
BTW, if your fuel burn gives you that 10 to 12 hour running time I suspect, and seas kick up turning that 2 hours to Catalina into 3 hours, having that forward tank empty could leave you short of getting all the way home.

That's 4900 lbs. sitting aft. Every boat's COG is different, so experiment, but I think you might find it better to take less in the aft tank than emptying the forward.

Errrrr aahhhhh?
It may not be all the temp of the fuel, the bilge area temp vs the water temp vs outside air temp and humidity are all factors in condensation forming in your tank. Fuel temp is a factor but vented empty tanks will condense and make water.

Uhh, I think these physics are way off. The volumetric expansion of air is almost 10 times larger than that for diesel fuel. A 40 degree F change in temperature will yield a less than 2 percent change in volume for the diesel but will yield a more than 13 percent change in air volume. The changing air volume will draw in and expel air out of the vent as the temperature changes. Of course the humidity and dew point will significantly affect the amount of moisture condensation.

Capt J my guess is there are a few things at play, that the sticking coefficients and wetting properties are very different between fiberglass and the diesel fuel surface. And this leads to a different evaporation energy. On the dry surface, the water molecules are not strongly bound to the surface, and only a moderate thermal energy is required for evaporation. It is quite different in the diesel fuel because there is also a solubility/saturation element that effects the total amount of moisture absorbed and further since water is denser than diesel as the water is absorbed, it sinks and is trapped in the liquid and builds up over time.

The end result is that the expanding/contracting air sucks in the moisture laden air and as this moisture condenses it is more easily evaporated from a dry surface then the water molecules that become trapped below the diesel fuel.