I'm not really the best person to be replying to this because I'm still learning things like that but my best guess would be that its the longer version of 一日中 as I've noticed that lots of words have long and short ver. to them ( as to be expected) but again I could be very wrong!
It's also like "day and night". Except that you don't divide a day into two part but 4. Most japanese definition that I found just say that it's the same thing as １日中 or いつも (with a few other adding 日夜, 終日 or 常に). So there is no real difference in meaning.
Otherwise, it's really identical to 24x7 (with the seven). Except that it's used for a day. 24hx7 = a week 6hx4 = a day
I guess it can give more "weight" or "emphasis" at most? But otherwise, there are identical from what I can tell
Edit: forgot to answer your other question but that ちゅう is most likely 中. And you don't always need に. I don't think there is a grammar point for that since it's just a suffix. Basically, it's like in the word 月内 (げつない, "inside a month"). And from there, you get "Within a month" or "by the end of this month" from the context and when you translate it more naturaly. It works for other period of time too (like 今週中, 今年中, etc). Apparently, there are exception though. But when someone read 中 as ちゅう, it will probably have this meaning (じゅう on the other hand would have the meaning described by マイコー)
The key here is 中／中, (じゅう), which means throughout. It does not mean a length of one day, but is used to refer to an action or state that exists throughout the entire day. So this would be used to say "it rained all day", emphasizing that the rain did not stop for the entire day.
It does not necessarily mean 24 hours. If someone says 一日中寝てたよ！(I slept all day!), they are not saying that they slept for 24 hours straight. It refers more to the "day" in the sense of "day" vs. "night".
