Thinking about battery storage for off-grid RV builds, what capacity do you recommend?

Happy to share what worked for us. I run a small cabin setup for my wife's home dialysis equipment, so I also treat uptime as non-negotiable. Our base load is close to 2 kWh a day, and we landed on about 10 kWh of usable storage, built out of two 48V rack batteries. That gives us roughly four to five days of quiet running with no sun, wich has covered every cloudy stretch we've had except one ice storm in January where the generator earned its keep.

I took the opposite sizing approach on my setup and I'm glad I did. I run a small food trailer and started with about 3 kWh of usable storage against a 2 kWh daily load, which gives me barely a day and a half. Instead of growing the bank, I added two more panels and let the generator cover the odd three day stretch of clouds. Running it for an hour or two each afternoon burns maybe a gallon of gas, which is far cheaper than the extra lithium I was quoted.

The thing with steady low loads like yours is that the battery rarely cycles deep, so a smaller pack ages slowly. My cells are still above 90 percent after three years. One thing I'd flag for your trailer build: keep the charge controller and inverter out of direct sun. I cooked a cheap controller the first summer before I figured that out.

For steady loads like a mini split, you really have to think about the whole system efficiency. We were looking at a similar medical setup and the thing that changed everything for us was using thick copper busbars for the DC side connections. It made the entire electrical arrangement run much tighter and eliminated any heat concerns. It felt like a small change but it totally stabilized the power delivery under constant load. Are you getting any kind of detailed load calculations for the mini split's compressor phase?

I found that planning for a full week of no sun is overkill for my specific situation. My setup uses a 3 kWh bank for a 1 kWh daily draw. I sized it based on the most restrictive weather pattern I expect, not just typical cloudy days. I added a buffer because if the load jumps even a bit, that extra reserve really helps the system stay reliable. I like to think in terms of months of operation, rather than just consecutive days without sun.