I think it still makes more sense that it was the murder site before a dump site nevertheless -- for the plain and simple fact that the Mississippi River was a couple miles away from West Memphis, if that. My point is: if one was going to dump a body, they would have done it there -- assuming they had even a cursory knowledge of the West Memphis area (which, if they knew the victims, they would indeed have such knowledge).
The boys were last seen heading into a section of the RHH. They were seen by Cindy Rico on the north side of the bayou around 6 p.m. My point is, they were all over RHH. As Fr Brown mentioned, a killer would not abduct the victims from an abduction site, only to return them to that same site to dispose of their bodies -- I mean, it simply makes zero sense to do so. If you want to conceal bodies, you wouldn't return them to where they were last seen; and you wouldn't risk returning to the abduction site and being seen yourself.
With the exception of maybe the knife/weapon wounds (which do exist by the way, even if you do think there was animal predation -- there were what appeared to be box-cutter marks on MM's scalp, etc.), everything about this crime was improvised. To me, this improvisation would also include dumping the bodies. This dump site was used because it was the only thing available at the killer's disposal -- because it was in the immediate vicinity of where the crime/abduction had occurred. The water level in this creek was a mere few feet -- again, this wouldn't have been a convenient space to hide bodies (you would want deep water) -- and it was well-known that the water level fluctuated even lower than that at times. It was used out of necessity; not by choice. The killer needed to dump the bikes in the bayou and risk the chance of being seen because they could not fit in the creek where the bodies were found -- it's just another sign that this dump site was the most inconvenient of places to willingly dump evidence; yet it was indeed used, because the killer had no other choice.
Yes, you make the point that the killer would have to be seen exiting the site all the same; I agree. That is why I believe that the killer(s?) were on foot. Remember: there were zero tire tracks found around the scene -- and being on foot would have made the killer less of a target than being in a big vehicle.
The truck theory is a good one and I can't discount it. It would explain why and how the killer was able to contain three kids (in the back of a semi-trailer); and it would explain how all these items were able to be disposed -- because the killer had a view of when the searchers were out of the area, and disposed of everything over a prolonged period of time.