Most trucks offer some form of auto/load leveling for trailers or even heavy loads in the back of the truck (especially those using air ride systems). I'd be curious though, if it offers a side to side leveling or just a rear load leveling capability.Rivian has said in some of their photo and interview comments that it has self-leveling for towing and also useful for the bed tent. It's "automatic" so I wouldn't be surprised if you don't have to do anything special. Some pictures have shown a ride-height adjustment in the infotainment screen so you can set one of about 5 different heights.
I would think it would be a challenge for the truck to level in such a way, but it would be a nice feature, that's for sure.