Check your state engineering rules and laws. If it is engineering, and if there's not something that exempts it, then it should require a seal, whether the owner or building permit requires it or not.
A lot of the tank owners would be covered by industrial exemptions for their own work, but this doesn't usually extend to outside contractors or consultants who do engineering for them. Because the owners are exempt, in many cases, they don't care whether you seal things or not, but the state laws and rules are still there.
In Nebraska, perhaps some other states, a PE consultant on a job can specify a product such as a tank, and shop drawings and related engineering can then be done without a seal.
