If you indeed have the unit you describe (although the Freedom 458 series from Xantrex comes in 2000 watt and 2500 watt modified sine wave models, but not 3000 watt) it is an inverter/charger
. This is both a highly accurate battery charger and an inverter.
Do you have 50 amps in your trailer? How is your inverter connected? Does it have it's own 30 amp power supply at the breaker box? Is the charger section even being used?
These type of inverter/chargers have a pass-thru transfer switch that energizes the 'inverter circuits' whenever you are connected to shore power (assuming that your inverter is properly wired, which seems to NOT be a valid assumption) and will not invert when 120VAC input is sensed, being in standby mode.
There is an on/off mode for the inverter on the remote control panel (I assume that you have one). Check in your Xantrex manual for the proper operation of the remote and it's charging and inverting functions.
You don't need an Airstream-specific electric tech. In fact, most Airstream techs have little to no experience or knowledge of full-function inverter/chargers and their proper wiring as these are generally found on motorhomes.
Find a qualified tech with experience on AC and DC circuits who is intimately familiar with inverter wiring. It sounds like you have a real can of worms on your hands!