The RV in question is not mine but belongs to a semi disabled relative full time RVr..the solar got stolen and honestly they always park in the shade
so its not worth doing another one. It was made by Thor I think about 10 years ago. Its slowly falling apart as they do so systems are needing replacement. I put in 2 100ah Weisse VLRA batts in parallel and they are getting charged slowly when the genny is run but that's inefficient. The gen is an Onan 5500,standard on so many RVs.
One thing...as far as running the Onan 5500 and then just using an AC plug in charger ....might the batts get too much of a charge because the onboard charging system will be charging the batts whenever the generator is running.. so the batts would be getting a double source of charging...what do you know about that? If indeed that is a problem then the little Generac 2200i
for 600$ would be useful on its own just for charging batts. It would use much less gas than the Onan 5500.
If it were my RV and I was a full timer I would put in 2 eg4s and a suitable generator and controller for those batts..I would rip out all the rats nest of wiring onboard and start over with a high quality transfer switch and all else that was needed. And I would put 500 watts solar on the roof.
Is this a Class A,B, fifth wheel / toy hauler or bumper pull? I ask such as in a Class A,B or toy hauler the Onan is drawing from an internal tank and would be foolish to have an external generator with an external fuel need. Even more so if disabled and full time. Granted, never seen a bumper pull with an Onan 5k.
Either way none of these have an actual battery charger they just have an AC/DC convertor spitting out about 13-13.6 volts. It is located in behind the fuse panel in the lower half, upper half has the transfer switch. An external generator with DC would poise the same situation, just voltage and not a true battery charger.
With the assumption that with the Onan 5k the 12v DC convertor is at least 30amps, not sure why it would be charging slowly. If you want faster than 30amp then you will need a complicated two bus system that will isolate the batteries.
Can you confirm the batteries are not drawing the amps to charge?
You have the right components, replacing a $5k generator with a $600 generator is not the right approach. Need to look deeper into why the batteries are not charging. Lead acid or AGM will not get too much of a charge for the most part. Chemistry is your friend here. 99% of the RVs you see work just fine as such. Only issue I could see if you have a 3-way fridge onboard (AC, propane and 12v DC). Rare, but they do have a considerable constant draw.
In my 5th wheel my Onan 5K has 60 gallons of fuel to pull from. Why would I want to put an external generator on that I need to fill every 8 hours or so.
Transfer switches in RVs are just dry contacts, proven for years. No need to upgrade.