The FX only draws HALF the power of the bigger bikes.
a 2500 inverter will work perfectly. I run my house
off of solar panels and batteries and charge my
Zeros ( FX and SR ) using an inverter. You will have
to be aware of the voltage in the battery during
charging. As far as 50% goes, an AGM battery fully
charged is 13.8 volts. At 12 volts it is 50%, so don't
think of 50% being 6V. Running the generator should
also do fine, but kind of defeats the purpose of
electric propulsion. You could do this with solar
panels charging the batteries as you charge the
FX but would probably need the entire roof of your
motorhome covered in panels. You might also
bring / create an adaptor for the plug. Some
people have issues with Zero Chargers and
some A/C outlets. I believe that my FX
charger is okay but my SR sometimes has
issues with Chargepoint systems