Interesting question. I looked on my home state, Oregon, and this is all that is said.
"A tank vehicle endorsement authorizes the operation of any commercial motor vehicle carrying liquids or gaseous materials. A tank vehicle endorsement is not required for portable tanks having a rated capacity under 1,000 gallons." So it would appear not.
I looked on the FMSCA site and couldn't find anything that answered the question. I do know that OR follows the Federal guidelines for driver regulations, as most states do.
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