The one UNIQUE function which a mupltplus can provide is this: Augmenting inadequate grid or generator power coming in on the 30A power cord.
(If the voltage drops from TM 120v loads which the "grid" supply cannot supply, the Mulitplus can stabilize and augment the incoming grid 60hz supply with synchronized inverter power, obtained from 12v (batteries and/or solar).
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A configuration like mine cannot do that. '120v' is supplied from EITHER the grid *or* the 12v system, they cannot be combined to provide more power.
But it costs less, and the OTHER separate components are each replaceable as individual units. Here are the 3 other functions which the Multiplus provides:
(1) an automatic transfer switch. A 30A switch is adequate, and a really good one costs about $140 delivered (the progressive dynamics PD5110010Q, the being new version with WAGO-style connectors).
(2) A high quality (and bigger) Inverter. My own Inverter (this one:
https://www.ebay.com/itm/39483776702...8AAOSweW5U87Fn) cost less than $180, although I also bought a couple of other bits to make a low-cost remote cable. An even bigger version might fit the compartment at slight higher cost.
(3) A 120v -> 12v power converter (replacing the WFCO "main board'). There are multiple options ranging up to about $400. The highest of those prices consists of a set of TWO "30 amp" from roughly $300 for a high-powered non-programmable model (progessive dynamics), up to about $450 for pair of cellphone programmable Victron Energy "Blue Smart IP22 12-Volt 30 amp 120VAC" battery chargers with built-in bluetooth.
It is possible that just a single "30A" Victron will exceed your needs. You could try buying just one and adding the second later.
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In all 3 cases, separate components cost less and provide higher servicability.