As posted above, wiring after the ATS is common to both shore power and generator power. The 3000 XC has a built in battery charger and its own transfer switch to switch between battery powered 120 volt output and pass through 120 volts. The XC will pass through 50 amps at 120 volts.
When shore power or generator power is not available the XC will switch and draw 12 volt DC power from the battery bank and output 120 volts.
Q1: Does your RV have a 30 amp 120 volt shore power system, or a 50 amp 240 volt split phase power system?
Q2: Does your generator produce 120 volts or 240 volts split phase? Onan makes many versions of 10,000K generators. Some can be configured to output 120 volts single phase.
Q3: The XC should output 120 volts when shore power, generator power, or battery power is available. You said your system outputs 120 volts on shore power, but not on generator power. Does it output 120 volts when using the battery and neither shore nor generator power are available?
Q4: Does the XC display panel light up? What does it say? Is there an error code?
Assuming the shore power is 50 amp 240 volt split phase and the generator is 120 volt single phase, L1 and L2 of the shore power are hot. L2 of the generator power may not be. The XC has only one set of 120 volt input connectors. It may be connected to the L2 which has no 120 volt power. This would be uncommon.
“On generator power I get power up to the inverter but not out of the inverter.”
Q5: Did you measure the voltage on the input connectors of the Xantrex? If not, where exactly did you measure the input voltage?
Q6: Exactly where did you measure the output voltage?
Half of the RV outlets may be powered from the Xantrex. The other half of the outlets would have to be powered by the generator directly.