I would recommend plugging it into whatever you have available so you don't come back to a dead Rivian. But it would be interesting to see some long-term vampire drain data. Just my 2 cents.
I believe “they” say you should plug it in and leave it set to keep 50% charge…
You will loose ~1.5% per day (average) and depending on how long you are away, you don’t want the battery to drain completely…
You may want to explore putting it into storage mode or transit mode, I’m not sure the labeling. The benefit being that it will draw less power. don’t forget that if the truck has network access you can monitor it’s charge level and remotely enable charging.
I would set charge level to 60%, plug it in to keep it charged, and... connect a 12V trickle charger (check to see if you have a single or dual 12V battery model, IIRC the cutoff for the dual battery system was May 2023 BUILD date or thereabout).
It's been proven over several months of trials by members including myself that keeping a trickle charger connected to the 12V batt(s) drastically lowers vampire drain 1/4 to 1/5 of the level it is without the trickle charger. It also will prevent dead 12V batt(s) upon your return months later
My setup is below, you can forego 95 % of the work I did because you will only be using the trickle charger one time, not everyday use like I did. I went to great lengths to conceal my setup, you won't have to do any of that.