I'd love a system that can link everything together and figure out the best (and most cost effective way) to get power
If there was a fancy box I could link Solar, PowerWall, Generator and the Grid into.
Then you could also set it to do things like charge the PowerWalls from the Generator if needed to have power at night, and have the generator turn off. Don't think anything like this exists though
I can't see in theory why that wouldn't be possible and why any specialist central control would be necessary. I watch a lot of Tesla Powerwall stuff and the controls look for a failure of the incoming supply and then switch seemlessy over to battery and then back when power is restored. By the same token, it's possible I believe, the guy with the Powerwalls above will know, to control and charge the batteries using grid if required.
BTW, not all power wall installations that are grid connected can provide back up power if the grid goes down. It's a feature on newer units as prior to this, the Powerwall had to swicth off to keep the grid lines safe for mainenance engineers - the danger was they switch the grid off, and all the powerwalls feed the grid and make it live again potentially electrocuting the engineers. For this reason I believe many grid connected powerwalls from ealier installtions cannot supply the house with power if the grid fails despite having stored energy. I believe Tesla made changes to the control units in later grid connected models to ensure if the unit switches to battery backup it isolates itself from the grid making use as a power fail backup possible.
Anyway, going back to the topic, if the power wall controller just sees power coming in, it's not going to know where it's coming from, so I see no reason why you could not have a generator upstream of it mounted as the 1st device on the supply that kicks in if the grid fails as I don't see how the powerwall would know it was the generator and not the grid supplying the back up power. It would potentially just switch power to powerwall supply between the grid going down and the generator kicking in, then back to "grid" when the generator came in. I do wonder though if the generatoe might be better downstream of the powerwall so the power wall supplied power first and the generator second, just simply because you presreve the fuel that way. I defer to electrians on the actual possibilities.
One last point, be safe with propane heaters guys. Carbon monoxide is a real and present danger indoors. I suggest getting an alarm in every room where one is used.