Technically it's best to use the + post on the first in the bank and the - on the last, but the difference is largely provable only in a laboratory. For very large banks, the best points are more like the middle than either end, but few Rvs have that many batteries. The reason for not worrying much about it is that several other variables can and usually will easily overshadow any difference in attachment points. The average wire length from the power source to each battery just doesn't vary that much in most RV layouts. I'd let the physicists worry about it and just hook up to the most convenient plus and minus terminals.
