We often get this question during audits and PHAs, as it seems to be “common practice” for the driver to reside inside their cab doing paperwork while they are the intended “hazmat attendant”. And although it may not be perfectly clear under DOT regulations (I think it is clear there as well, but many will argue otherwise) I turn to my favorite code, the International Fire Code (IFC) which states the following… (emphasis by me)
5706.6.1.2 Leaving vehicle unattended.
The driver, operator, or attendant of a tank vehicle shall NOT remain in the vehicle cab and shall NOT leave the vehicle while it is being filled or discharged. The delivery hose, when attached to a tank vehicle, shall be considered to be a part of the tank vehicle.