Onan has a generator controller similar... The EC-30W.
When I was working on a Transit build (now cancelled, mainly since a family member feels too cramped in a "B"), one of the items I was considering doing was going with a 24 volt battery bank, and a Magnum Energy hybrid inverter that takes power from two 120VAC sources, either one by one, or both running. A nice bonus is that if there is a brownout, the inverter will boost voltage, using the batteries to compensate.
This is about a $3000 solution, factoring in the inverter, a generator controller, a control panel, and a battery monitor... but it will allow for seamless transitions of power between links.
