Based on what I've red, most red fuel is actually ULSD. Not because it has to be, but because refineries don't want to make two different kinds of Diesel fuel.
Of course, if that statement doesn't turn out to be true, then you can legally run red if you keep a log book and mail the Fed's the taxes for it at the end of the quarter.
Also, I think that directly taking fuel from an in-bed tank is of questionable legality, which is why a lot of people run the transfer tanks and just pump it from the in-bed to the stocker. And, as has been said, it also might be illegal to carry over 100 gallons of fuel, especially in a non-DOT certified tank.
AND TO ANSWER THE QUESTION:
The Standard Method
I have a dual tank setup myself, so I did research when I set mine up, and it seems that the only thing available is the Pollak 6-way valve used in Ford's OEM applications. To my knowledge, they're the only guys that make valves for this application.
http://pollak.thomasnet.com/viewitems/f ... &forward=1
The downside to them is that the port sizes are a bit on the small side, and that they cost $70.00 from the eBay resellers; however, I got mine from a distributor and it only cost $35.00. (The other downside is that they apparently don't like hot fuel as much as they say, so I'll have to get rid of mine before I finish my veggie system.)
You splice the Pollak valve into your stock return and sender lines, then you run your auxiliary return and sender lines to it. With the flip of a button, it switches both lines simultaneously, avoiding cross-tank contamination. It even switches the fuel tank senders so your gauge reads whatever tank it's feeding off of.
As for me, I'm passively looking for a pair of heavier-duty three-port switches with 1/2 inch diameter, so I don't have the restriction in my fuel system or melt the Pollak with my 200 degree oil.
1990 D-250 Regular Cab: Tweaked injection pump, built transmission, a cataclysmic charlie foxtrot of electronics, the most intense street-ran water injection system in the country, and some more unique stuff.