The "pure-sine" thing is probably an inverter, perhaps an inverter/charger depending on the particular device.
If you're planning to use the inverter to power all of the 120v system when you're not on shore power, and shore power when you are, I think you'll need a transfer switch.
In any event it sounds like you need to consult with someone who has experience with this sort of mixed system. The way new Airstreams are set up is that certainly outlets are supplied by the inverter, and I believe the inverter has a "pass through" mode when shore power is available so those outlets switch over to shore power. The other, normal outlets are only powered when on shore power or a generator. Since the trailers use a normal inverter (rather than an inverter-charger like the Interstates) they may use a transfer switch for those, or may just power them with the inverter only. Putting in dedicated inverter outlets would require running some new romex, though.
Zero Gravitas 2017 Flying Cloud 26U | Il Progetto 1976 Argosy 28 Center Bath | WBCCI# 15566
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