The math is correct, except the problem has a flaw or two,
I build gasifiers, and have run two 1/2-ton trucks on "producer gas", (Hydrogen derived from biomass) this is NOT a "supplement fuel" it's a replacement to gasoline (see the link below). I start on it, run on it, and finish on it, BUT biomass is WAY different than getting H2 from water using electrolysis.
Three buddy's of mine are truckers, and 2 have a very elaborate systems and get supplementary fuel for their big trucks using them, they do NOT replace diesel, just help it when up to speed on the highway (literary "pouring on the throttle", lol)
H2 from water is NOT a replacement, IMO, instead it's a very viable addition, easy to make reliable systems can be found on youtube.
personally, since I run the other trucks on producer gas (syngas) and have a vast supply of biomass to run from, I'll continue down that road.
Here's the link to the "quackery" that I do, called "Biomass Gasification"......