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Without details, it is really hard to say whether that statement is true or false. A lot depends on where you are located (solar insolation), how much power you are consuming during the day (produced by the panels but being used instead of put into the battery bank), panel efficiency (how they are mounted), etc.
In rule of thumb terms, you should have a solar array that produces 13% of the total battery bank capacity. A 400 Ah bank would be matched to an array that is producing 52 Ah (of charging current so up around 14 volts). That would be something around 800 watts but have to take losses into account. All that said, you need to fully charge your battery bank. Insufficient charging (less than 100%) will, given enough time, damage the battery bank. All those numbers work fine for a 'fixed' installation (off grid home, for example) but for an RV/bus, they don't work so well. Sometimes due to insufficient space on the roof for panels, moving from high insolation area to low, shading (mountain/trees), and varied electrical consumption. Many folks use a generator to fill in the void. I'm not a fan of generators but I get that they are sometimes necessary. Run it for a bit in the morning to bulk charge the battery and then let the solar complete the charge over the course of the remainder of the day.
Depending on how you mount your panels (flat or tilt) and where you are located, you'll likely see something between 50-80% of rated output. So, an 800 watt array may only produce 400 watts which is probably not going to fully charge your battery bank (if it is down near the bottom (50% SOC).
"Networking" charge controllers is common to support a large array. One way you can "cheat" is by going with a higher voltage battery bank. Most charge controllers are rated by output amperage regardless of voltage. So, a 60 amp controller will output a maximum of 60 amps to a 12V battery, 60 amps to a 24V battery (twice as much power) or 60 amps to a 48V battery (four times as much power). There are cons to the approach, of course...