When (according to about:unloads) my average firefox tab is 70-230MB depending on what it is and how old the tab is (youtube tabs for example bloat up the longer they are open), a chat app using over 350 is a pretty big deal
just checked, my firefox is using 4.5gb of RAM, while telegram is using 2.3, while minimized to the system tray, granted Telegram doesnt use electron, but this is a trend across lots of programs and Electron is a big enough offender I avoid apps using it. When I get off shift I can launch discord and check it too, but it is usually bad enough I close it entirely when not in use
To be fair I think it is there as a backup for low temperature climates, the Lithium batteries wont charge at temps that low, but they still could have setup the lithium batteries as an emergency backup for all the 12v stuff.
Cellular is usually how the vehicle provides Wi-Fi, it is effectively just a cell hotspot like you would get from a ohone carrier, but tied into the vehicle. So I think that would be the common way they get the data out.