Well in Iowa you don't have to cross scales without a commercial plate either but atleast once a year I have seen IRS agents running around with DOT officers stopping any truck that is a diesel, and testing the tank $1000 first offense $10,000 second offense in one year. And yes I said IRS cause it's considered tax evasion which means you are guilty, and they don't have to prove your guilty beyond that.
Also for those that run an offroad tank in the back you would be suprised how many try hooking that into the fuel system, guess what the IRS agents know that trick, and have been know to pull some fuel out of the fuel filter if they suspect your doing that (and having totally cold fuel in a tank on a running truck is thier first clue yes they sometimes check fuel temp too)