"How can I find out more about this charger issue - is it documented elsewhere"... yes in your manual, and most sellers have big red writing on their advertisements to this effect.
here is a cut and paste of one from the second in the list from ebay today...
http://www.ebay.com.au/itm/3000w-120000w-LF-pure-sine-wave-Power-Inverter-DC-24v-to-AC-240v-converter-H5-/261920139035?pt=LH_DefaultDomain_15&hash=item3cfba8271b"
PLEASE NOTE: for battery charge mode: before charge the battery,
please turn off the inverter first, connect the AC power cord with home AC source and inverter, then turn on the inverter to start the function. When finished, need to turn off inverter first, then disconnect the AC power cord. Otherwise the PCB board might be damage due to high voltage spark."
You will note in the same advert, that they claim that is fine as a ups.... ... don't believe them, they cut and paste bits of each others ads from the look and have no concrete idea what they actually have for sale sometimes... guess they are only salesmen/women, and have never used one.
So very good inverter as an inverter only, or supervised charging.
So I don't want to see you running a PJ as a ups for that reason.... regardless of what they don't say in some adverts.... until they claim to have solved this problem explicitly with next years model blurb eg.
So you can use either one or two chargers, banks connected together or separate... in the end it won't make much difference if your only using it every few weeks. Two gives you redundancy.... which I like. Common earth is fine.
Yes the alternator is only a charger, so if it has no problems with however you set your isolator switch, nor will the charger/s. The alternator is after all only a charger ......diode isolated from the batterie/s anyway. via that isolator....
So you can connect as many chargers as you please to a single battery, or a single charger to several batteries without disaster. The 20a charger is technically big for the starter battery as it exceeds the C10 rate... but won't ,matter as the battery will likely be full when you get port anyway, so will go straight to float in minutes... and the house bank is technically under done with 20A, but won't matter either, because the long recovery time, so it won't get to C10 rate...... but so what........ really, you have time on your side which is even better for it.
I suspect the fridge is only light % duty, so the 20A will be fine.
So I would be happy with the two chargers, however they were configured, so long as they were a little smart... ie had at least two stages, bulk/absorb and float.
The PJ
It is from the reversion to default frequency from the mains frequency. If they differ in waveform/phase/timing call it what you will, the surge current into the transformer can be huge. These boards were designed with E I (normal) transformers in mind, and work perfectly with them, as their leakage is high enough to absorb the surge, torroids are very stiff comparatively, so start currents can be up to 60 times the idle currents... so it won't blow every time, maybe 20 percent of the time...so plugging in the shore is not a problem ( power jack advert wants shut down in each direction to be safe), as the device syncs to the mains before changeover, it is the turning off and reverting to default 50hz thats the problem, there is no smooth transition programming.
..................oztules