Okay, the best first step would be to see how many watts you use per day (24 hour period). You might assume that the fridge is going to use about 1,200 watts per day. Lights and water pump will be fairly nominal; but depends on how much you manage usage. If you need heat, the furnace will use some capacity and depends greatly on temps of course. I assume your RV came with a solar panel for the fridge and likely a shunt/battery monitor? If so, you can use that to turn devices on and see how much power they are consuming and estimate daily needs. All of that will help you figure out approximately how big of a battery bank you might want.
Regarding the Bluetti...there is absolutely nothing wrong with a system like that. It's convenient, plug and play, everything is put together for you, it's expandable if you decide you need more capacity, etc. Some people don't like them because dollar for dollar, you can usually get more capacity, higher output current, a solution tailored for your specific needs, and often better components, if you put together your own system. But that requires you to either spend the time to research, buy the components, and build and install the system, or pay someone to do it for you.
For what you need now, and assuming you don't really want to build your own at this time, it looks like an off the shelf solution would work well for you. If you did want to start with a simple build your own, I would say buy a couple of portable solar panels and charge controller (if the panels don't already have them built in). Wire the charge controller to a port you install on the camper. Get long cables for the panels with a connector to plug into the port and start with that. If you decide you need more capacity down the road, add another battery.
If you decide to go that route vs the Bluetti, you might consider replacing your lead acid with a larger LFP battery vs a 100AH. For the same weight or less, you can get one in the 200AH range. If you decide to add another down the road, you can double that capacity and still only add about 50# to the RV.
Apologies for the book...