What is formula to determine how many watts per amp. I have an RV with a 750 watt inverter and want to know how much wattage vs volts the inverter is charged with my Onan 5500 generator or my Honda 1000 generator

Still don't quite understand your question, maybe you would like to elaborate?

The formula I think you need is:

For ac systems, Apparent Power (W) = Voltage (V) x Current (A).

The values of Voltage (V) and Amps (A) should be Root-Mean-Square (RMS) values.

An inverter takes the 12 volt from the battery and converts (inverts) it to 120 volts, 750 watts of power at 120 volts = 6.25 amps.

A generator(actually an alternator) gives 120 volts output. A 5500 watt genset can provide 45 amps,usually two 20 amp circuits or outlets at 120 volts. If the Honda is a 100 watt unit, it provides 8.9 amps at 120 volts.

Not sure what you mean by "the inverter is charged with my Onan 5500 generator", the inverter and generator are not related to each other, as each is intended to provide 120 volts AC power.