@Daiki Jin
Not knowing your trailer dimensions and actual system layout, it's hard to say if you'd benefit from going to 24-volt, considering such low demand. Still, the price of copper wire has increased significantly since I refurbed my camper back in December 2021 through January 2022. I had bought a lot of components before then, and lucky thing, too, considering inflation. That said, I think a year from now, you'd wish you had gone to 24-volt.
As to budget, you don't need the newest and best of the best and, therefore, the most expensive, nor does it have to last ten years; newer and better tech is always just around the corner. It's a costly illusion; don't fall for it. From reading the thread, it seems you've already gained the experience of building your battery, as did I, but with the cost and quality of ready-made batteries, it doesn't make sense. Used solar panels are dirt cheap and perfectly good. I bought nine used 250-watt panels for $65 each last year, and they're working just fine. I have them in three arrays, each array managed by an EPEVER 4215N 40 Amp MPPT. One is for ground deployment.
Since you're still designing and budgeting, I'll tell you what I did and why instead of what you should do. You could get by very well with half my system. You seem to have the basics down. A couple of years ago, I found a deal on a 1988 27-foot class C on the Ford E350 platform. It only had 53K on the odometer, ran perfectly, and everything worked; $4k cash; I couldn't pass it up. It was old tech, and over time I've upgraded, and one thing was a new 15K BTU A\C unit I found on sale. The RV is about 160 square feet, close to a 20-foot trailer. Now, like you, I have a window unit, a 6K BTU GE with a thermostat for boon-docking, which will do the job, but the overhead unit is for running on shore power. I found a nice ten cu foot apartment-sized fridge to replace the old 12/120v/propane unit. It was a shoe-horn fit but worth it.
I was browsing Will Prowse's forum/website when I found a thread from a Chin factory rep in Northern California in November last year. These batteries were selling on Amazon for $650 at the time with free shipping because of my Prime membership. She returned with a bid of $600 each with free shipping to my door. I couldn't beat it with a stick. I went with Chin 12-volt 200 Amp-hour LiFePO4; four configured 2S2P for 400 Amp-hours at 25.6-volts. That's 10.24kW of storage. You would be hard-pressed to build a 12-volt 100 Amp-hour, including a 100 amp BMS for $600. And no, I could give a "flying blank" if they're A or B-grade cells, they are great batteries, as are some others just like them. I know several people who did the same at different times, and all like them and have no issues. They came fully charged, packaged well, and had all kinds of documentation. They even threw in a sweet 15-amp charger.
I went with the Giandle 24-volt 4000w pure sine wave unit for my inverter. During the day down here on the beach in East Texas, with all three solar arrays charging, I can run the overhead 15K BTU A/C with no problem during the day and throw something in the microwave simultaneously. She doesn't blink. Flat out at 4000 watts at 120 volts; you only pull 165 amps from the batteries on the 24-volt side. Do the math, and you will quickly see the amperage at 12 volts. At 12 volts, you'll never draw the total 1280 watts from a 100 amp-hour battery. Because you can discharge at 1C doesn't mean you should in my book.
I'm an old retired engineer who likes to live comfortably out in the middle of nowhere away from people with all the comforts of home. I'm in Houston now, staying with my brother and doing upgrades. Gas is a bit costly right now, especially with a big block 460. I'll post pictures sometime when I have her looking pretty. She's a bit rough around the edges right now, but so am I. At my age, I've earned it. I've built a 3-foot rear bumper extension rear patio with all kinds of useful crap, including a sweet 4300w on gasoline and 3800w on propane duel fuel generator and slightly used on eBay for $500. My toad is a 2000 WJ Grand Cherokee with the Selec-trac 4x4 and 4.0 liter six; I love my Jeep.
