I've got an idea. Get rid of the diesel altogether. If burning hydrogen with oxygen to get water returns more energy than it costs to separate the water into hydrogen and oxygen, then there's no reason to involve diesel at all. Just set up a big ol' HHO generator and fill the gas tank with water.
Even assuming a 5% concentration of H2 did everything claimed for mileage and emissions:
A duromax diesel displaces 6.6 liters. Even neglecting the turbo charger's boost, that's 5.28 cubic meters of air per minute at 1600 rpm. At a 5% mix as suggested in the above emissions test, that's .264 cubic meters, or 264 liters, of hydrogen per minute used in the test as a "catalyst". A mole of hydrogen is 22.4 liters at STP. So we're talking about 11.78 moles of H2 per minute.
The energy required to electrolyze one mole of water is
237 kilojoules. So 11.78 moles requires 2791 kilojoules of energy - call it 2800 kilojoules. That's 2800 kJ per minute. To figure out the wattage, divide by 60 to get 46.7 kJ per second, which works out to 46,700 watts. Which, at 14.4 volts, is 3200 amps.
The alternator on the duramax trucks is 145 amps. Even dumping 100% of the alternator's output into the HHO generator, you could only produce less than 1/20th of the required 5% H2. Plus you'd run your battery down in 15 or 20 miles.
Incidentally, 46,700 watts is equivalent to 63 horsepower. Never mind whether your alternator could make that wattage, your fan belt couldn't deliver the power in the first place!
So it doesn't matter if HHO works or not. Your car doesn't have the ability to make enough hydrogen anyway.
Incidentally, hydrogen isn't made commercially by electrolyzing water. (It's too expensive.) Commercial operations create H2 by injecting superheated steam into natural gas. The reaction is CH4 + 2(H2O) -> CO2 + 4(H2). the H2 is compressed and bottled and the CO2 is sequestered underground.
Really, really useful unit converter:
Unit Converter - Digital Dutch Unit Converter