There are these things called cloudy and rainy days. There are also times on the year where solar in the Northern Hemisphere produce very little solar due to low sun angle and shortness of the day. November December January February being the worst 4 out of 12 months for power production. During those months, the DC DC charger is your best friend in the world. Everytime I take a drive, the truck alternator charges the house batteries. In summer, I can switch off the DC Charger if needed. The solar is at peak performance 3 to 5 hours a day. After that, any assistance is desirable. The BMS manages the charge of the batteries flawlessly. Too much power has never been an issue. Too little power, that's a problem I've never had. The Victron units both sense the state of charge of the batteries and adjust their outputs accordingly.
I guess I never see that issue with my camper as I have a 9 day battery capacity and over sized solar which will replace 3 days use in one good day. I do have a 20 amp dc to dc but I have never had to use it.
We have 400 AH lifepo which does a marvelous job of running things. I can easily run a week if I'm only running lights and fans. I'm not doing that. I'm running a fridge at 30 amps, 2 laptops at 10 amps each, charging USB devices, charging camera batteries. I put my batteries, solar, and dc dc alternator charging to work everyday. We've been on the road about 2-1/2 months now and perhaps another to go. I've had days getting over 600 watts solar, and most days in the mid 300 to 400 watts, a few where my 800 watts of panels produced 40 watts. How you use your system matters. A nice side effect all those roof panels have is shading most of the roof area keeping it cooler in the TC. Im sure your solar works great in the peak of summer when you rarely need to run the furnace, but in winter, the dc charger can be your best friend ever keeping your batteries charged.
