This looks like a good plan. It's very much like my inverter/converter setup, except mine is not dual voltage. It should work fine within a couple of limitations.
The Onan transfer switch does not backfeed generator power through the 120/220 volt transformer to the 220 volt circuit panel, just as it isolates a 120 volt shore power cord from generator power. I think we can assume the feed-through transfer switch on the inverter does the same - passing shorepower to the 220 volt outlets when it is available and disconnecting the inverter from the 220 volt shore power circuit panel when shore power is not present. This means the inverter will not backfeed the 120 volt buss or the original converter - all it feeds are the 220 volt outlets in the rig.
The inverter will provide power to the 220 volt outlets with or without the generator running, within the capacities of the inverter and batteries. You will have to run the generator to use the air conditioner or microwave. The refrigerator will switch to gas mode unless you are plugged into shore power or running the generator.
You can use a combination inverter/battery charger with a built-in transfer switch if it's more convenient, but having the extra charging capacity may not be of much use. Since the input will be 220 volts, the only time the extra charging will be available is when you're plugged into shore power. If you're going to be plugged into electricity overnight does it really matter if it takes 2 hours or 8 hours to recharge the batteries?
You willl need a new, 220 volt inverter so yes, that does make your existing inverter redundant. You could salvage your existing inverter by putting a 220/120 volt transformer on the front (mains) side and another 120/220 volt transformer on the outlet side, but this is probably more trouble and expense than it's worth. And the inverter will put 60 Hz power on your 220 volt outlets, not 50 Hz.
When you're plugged into electricity the 220 volt outlets are fed directly from the shore power via the feed through transfer switch. But when you're not plugged into electricity, all of the outlet power comes via the inverter, not directly from the generator. This places a couple of limitations on how much power you can draw from the outlets.
First, the size of the new inverter will determine how much power you draw from the outlets. A 500 watt inverter won't power a 1500 watt hairdryer, for example.
Second, the inverter power comes from the batteries, which are in turned charged via the original converter. A 45 amp converter will supply about 500 watts. If you use less than 500 watts the converter will supply all of the power you're using. But if you use a higher wattage, say running a 1500 watt hair dryer, the excess power will be drawn out of the batteries and you'll have to run the generator longer to replace it.
I presume the author has a source for the "pass through transfer switch" that will be used with the new inverter, if it is not included as part of a combination inverter/charger.
The only real question I have is how well the 120 volt, 60 Hz components will work on the 120 volt 50 Hz power they will recieve when you're on shore power. 50 Hz transformers have to be about 20% physically larger than their 60 Hz only counterparts and I don't know if the 60 Hz equipment has this much fudge factor in their design.
A modern converter like the Progressive Dynamics should work OK on 50 Hz power. A converter with a 60 Hz ferroresonant transformer obviously won't. A 3 way refrigerator probably doesn't care since the only 120 volt component is a heating element. But I'd question the safety of running the 60 Hz motors and transformers in the microwave and air conditoner on 50 Hz power. It would be wise to ask the manufacturers whether or not their equipment can tolerate it.