Right side, left side, front, back...none of that is relevant. What IS relevant is whether the toilet drops straight down into the black tank, or does it have to make an angle (or two) to get into the black tank. So the location of the toilet in relation to the location of the black tank is important.
A secondary issue (which could be what the OP's neighbor is alluding to) is how long (and how many bends) in the black tank drain line (but that has nothing to do with toilet location); that can also cause problems when draining the black tank. There again, toilet location is irrelevant. Tank location in relation to dump valve location is what comes into play there. However, your black tank usage habits are probably more important; use plenty of water, septic-safe TP, don't leave valve open, dump only with a 3/4+ full tank, etc.
