Interesting. I had no idea diesel had a shelf life of over 5 yrs. And I'm rather surprised to find gas has a 2 yrs shelf life. I haven't done a ton of research, but I was always under the impression one year was about it, then the BTU's started to drop significantly. It would burn, but not as efficiently.
The one downside I see to his set up is volume. That's probably a 250-300 gal gas tank and 500 for the diesels. He should be good on diesel, but I can envision going through the gas rather quickly. Especially running the roads checking on the kids/grandkids after an emp.