Your configuration works, although the all-in one is stated to require mounting on a fire-resistant and nonconducting surface (concrete). Few if any RV "campers" have such mounting locations. Your equipment totals $1500 for battery, $750 for the combination charger/inverter unit, and another $220 for the "48V" -> 12V buck converter. And you have not yet addressed your "need" to charge a "48V" battery (with roughly 54.0 volts) from your alternator. DC->DC battery chargers are readily available for "12V" lithium battery banks, and a few models do "24v" battery banks - but devices which can output up to 54V for charging a "48V lithium battery pack from a 12v source are relatively less common and more expensive.
The actual size of your proposed battery is 100Ah * 51.2 nominal volts = 5120 watt-hours. If properly used (between about 97% maximum charge minimum state-of-charge not less than 15%) you will have a bit more than 80% available for use, about 4100 watt-hours. Your projected usage of 3800 watt-hours seems very close, for use of the battery by itself on a cloudy day.
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I would (and did) stay entirely with "12v" battery packs running in parallel. If you build or buy two "12V" 280Ah battery packs (they each have nominal capacity of 3584 watt-hours), you'll have 40% more battery capacity and no need for the 48V ->12V buck Inverter. You can simply spend the $220 from the unneeded buck Inverter to buy the 2nd BMS, wiring, and a 2nd fuse (and possibly a case) in support of the 2nd battery pack. 12V->12V battery charge units are readily available and cost less. Pretty good Sine-Wave Inverters with the same continuous/peak power specifications can be had at moderate prices, I paid barely $230 to have a good one delivered to my door (Ebay Auction) - and I tested it to full capacity for multiple minutes. You just need bigger wires between the batteries and the Inverter. (I have dual AWG 2/0 cables, and the cable lengths are short.)
The separate MPPT charger could be bought with smaller PV Voltage capacity but more current capability, with the 8 panels wired "2S4P" for an MPPT charger cable of more than 200 PV volts, or wired 4S 2P for the case of an MPPT with a lower PV Voltage limit.