If you have Ryobi 18V tools and batteries like I do, a small inverter will help power the trickle charger without running an extension cord. Jackery also make a small portable battery backup unit (I have the 2000W plus) that can be used. Works fine on the boat with three batteries to maintain. Just recharge it periodically when it runs down.
The parasitic draw from components would bring down even the largest 8Ah Ryobi tool batteries pretty quickly (less than a week I bet). A typical car battery already has 50Ah so an extra 8 will not add a lot of extra capacity. AT least beding a tool battery you can take them home and charge them but that's not much of a fix and forget solution but is a solution. Ideally you want something like solar to continuously keep up. Feed any daytime extra power into some battery and should last indefinitely given enough solar panel area/efficiency. Google around for a solar battery tender and something you like should pop up much cheaper than Ryobi tool batteries. There's also a lot of external battery power boxes on every YouTube channel and you can likely find something you can sit in your car or near it (theft risk) to power whatever other type of tender you want.
These cars have a small battery in the trunk. I find it hard to believe that is the actual battery for the starter, I had assumed it was a supplemental for auxilliary stuff... I can't find anything else in the manual so the battery in these cars might only be 20-25Ah capacity or some trivial little thing.
Sounds like they expect about 30 days to dead based on the manual:
STARTING AND OPERATING 141
...
EXTENDED PARK STARTING
NOTE:
Extended Park condition occurs when the vehicle has
not been started or driven for at least 30 days.
1. Install a battery charger or jumper cables to the 12
Volt battery to ensure a full battery charge during
the crank cycle.
2. Press and hold the brake pedal while pushing the
Start button once.
3. If the engine fails to start within 10 seconds, wait
10 to 15 seconds to allow the starter to cool, then
repeat the Extended Park Starting procedure.
4. If the engine fails to start after eight attempts,
allow the starter to cool for at least 10 minutes,
then repeat the Extended Park Starting procedure.
Maybe some day I'll get an ammeter on the battery and see what the parasitic draw looks like and what you'd need to size up for a solar battery tender.
Dart