(what's to keep somebody from getting a tax receipt for 50 gal of fuel and then keep filling their tank up until they get caught and then produce the receipts).
The same thing that keeps people from making a second tank that is not visible and running all the off road fuel they want. Nothing! If you only have one receipt, it better be relatively recent. If you can produce the forms that show you are paying tax on this fuel, and it appears that the amounts and dates are correct, you should be OK.
By the way, I use to be a tractor dealer and we advised everyone to purchase road diesel for their tractors. In our area, farming had declined to the point that many of the people who sold off road diesel did not sell enough to have good quality fuel. If you purchase road diesel, there is an IRS form that you can use to get all of the fuel taxes back at the end of the tax year. This way the tractor owners could run a better quality of fuel and it not cost them anything.
I had people tell me that they were going to keep the receipts for their truck and claim that on their taxes. I would always tell them that when they got audited and had receipts for $3,600 worth of diesel and then tried to claim that they ran it through a 30hp tractor, they were going to jail.
If you are legal, you are legal. I would not worry about it as long as I could show proof of what I was doing. Try to cheat, and you may pay the price.