Why I started with "Some rigs may vary, but typically"
To find out on a particular rig - have the radio on and clock set... Disconnect from shore power - no genny running (those bring the BCC or BIRD/Trombetta into play...)
Fully disconnect the coach batteries (use/store may not be enough - 'keep alive' power could bypass this...)
I expect in most cases the radio will go off... if so - main radio power is from coach.
Wait a minute or so - then reconnect the coach batteries... See if the radio retained time/date... If so - your 'keep alive' is from chassis... if not - your 'keep alive' is from coach...
(Of course need to alter sequence if your radio stays on with coach batteries removed.... or could mean your batteries are coupled when they shouldn't be as someone else saw recently...
Ain't consistency nice?