Ditch the Onan, house batteries in the trash, disconect the alternator and rely on solar only?
Ditch the Onan and use solar to suplement other charge sources?
Keep the Onan?
I had all the hardware to add solar to my coach, and I have added solar to 3 other camping rigs.
They can be as simple as a single panel and PWM charge controler added to an existing house system,
Or as complex as a series-parallel solar array running through a MPPT controller to a combiner and distribution block with LiPo house bank, dc-dc controller, Battery management system with bluetooth or wifi monitoring and charge/discharge controls, Automatic transfer switch(s), Shore power charger and inverter.
Heres the rough draft of the system I was going to put in my coach, it would be a bit more simplified if I wasn't trying to save a few bucks by reusing some onf the existing components.
As far as what loads you can run and for how long, that totally depends upon how much battery you have and how big the loads are.
I could run a microwave via the inverter, as well as lights, water pump, an ARB 12v freezer, and fans indeffinately.
The larger loads like the furnace fan, would depend upon how much direct sun I got. When the furnace is needed usually there is limited sun available to recharge the house battery banks.
When the fridge, freezer, fans are under heavy loads it usually pretty sunny out and recharging is not an issue.
I also supplement many appliances with propane, ie. Fridge, furnace, and water heater all run off propane.
AC is shore power or Generator only, even the modern effecient ones and mini-splits take an enormous amount of battery to run even for 6hrs a day. There are people that have done it successfully, but the upfront investment in solar and battery capacity is quite high. I don't know any exact numbers, but I would estimate $10,000. For reference, I'm into the above pictured system about $2000, not including the cost of the generator and the inverter as they were already in my coach.