"What we need is a method of connecting the US 240v four wire system to the European three wire system"
If your US loads are modest a good inverter , Trace 4024 (4KW at 24V input) will power many needs from a good sized batt set.
What US gear are you trying to power? For how long?
A dockside US power input and wiring for a few gadgets might be the simplest and safest .
With US 120 and 240 internal plugs and sockets very different from the euro style there should be little chance of a problem.