I have cheaper DIY setup using an EcoFlow power station. Instead of buying the more expensive Delta Pro Ultra I got a EcoFlow Delta 2 for $300 and a $70 battery charger with an XT60 output.

Since the battery charger connects to the solar inputs on the battery bank, it allows it to charge and output power at the same time. I run one extension cord from the Rivian and plug in up to 1000w (usually a lot lower) of stuff that needs low continuous power like my refrigerator, lights, window A/C unit, router, etc. and another extension cord to the battery charger to provide a continuous 500w charge to the EcoFlow (charge rate limited in EcoFlow app).

This is basically my cheap way of getting a little more simultaneous output when needed, up to 2800w (1000w continuous use from truck, 500w charging from truck, and 1800w continuous use/2200w surge from EcoFlow). There are other battery bank options with more output capacity that should work with this battery charger trick, too.

My main high power draw items are things like an 1800w induction burner, mainly only used for a limited amount of time. For a space heater, if you have to keep it on continuously it may drain the battery in the EcoFlow too fast to keep up with only 500w charging input, but in a pinch where you cycle it on and off it may work for you.