There is no doubt that the location and time of Huck Finn influenced Twain's depiction of the characters. This was a time period in which slavery was considered acceptable. There were definitely nasty owners who had fifty slaves or more, beat a lot of them and raped some of the females. However there were also some genuinely nice, decent people who owned a slave or two because that was what you did in this Southern society.
Location is huge too because the labor in southern states is much harder than that of in the north. Help is needed oftentimes and for that many people turned to blacks because they were easy to get and hard, good workers. Of course there were the lazy people too who decided to beat their slaves into doing way too much work while not lifting a finger themselves, but for the most part, people actually worked alongside slaves and just did enough labor for themselves and their families.
If it had been just 100 years later, it would be unrealistic for the characters to behave the way they do. Also, if they had lived a few states father north, it would be unrealistic as well. The time and location of this novel affects the mind set of the people a great deal.