Alright, as it stands SENSiT is a non-starter. The device is totally locked down and all traffic in/out is encrypted. Kingspan told me they have no plans to release an API to access the data outside of the app. They have subscription services that allow web access, but that doesn’t count for the SENSiT USB with app.
The communication between the sensor and dongle is over 433MHz radio, and you can pick up the packets with rtl-433, but no one has decoded them yet and they don’t match the format of pre-existing oil level monitors. So you would need to reverse engineer the packets.
I did try the daily email + imap integration approach but it fails to read the emails as they are HTML formatted. You could hack around this somehow I’m sure.
So I’ve abandoned this approach and gone with a different sensor using the method listed here: https://community.home-assistant.io/t/oil-tank-level-monitor-watchman-sonic-rtl-433-integration-for-rf-sensors-or-level-sensors/