Do intrastate tank truck carriers have to get a DOT number from FMCSA?
They are not required to obtain a DOT number by federal regulation, but they may be required to do so under state law if the state has adopted the federal regulations as its own. You can find out which states do require an intrastate carrier to register with FMCSA at http://www.fmcsa.dot.gov/registration-licensing/registration-USDOT.htm.
Intrastate carriers of hazardous materials ARE required to register with PHMSA. Check the HM registration information at http://phmsa.dot.gov.