Okay. I have been thinking about this for a while. I think ROTF Devastator is a Hexocon, like a Duocon, but made out of six. That is, he is one robot which turns into multiple vehicles. A similar example is Magmatron.
While this is not perfect, consider this:
[SPOILERS, I guess]
1. The specific vehicles are not seen in robot modes.
2. At the same time as Devastator's fight, Rampage is named by Starscream in the battle in the village, and seen in robot mode.
3. Mixmaster is seen in the battle at the village in his robot and third "weapon" mode (consistent with the Voyager toy).
4. Long Haul is seen in the village battle in robot mode.
5. When Devastator gets hit, he comes apart and stays that way, when from the look of the hit parts of him (if they indeed had robot modes) could get up and continue as they were not hit directly.
6. I think the names ascribed to them on the toy are just to try and connect to G1, nothing more.
7. I would be more comfortable with this if the cement truck part of Devastator (in the movie) did not have the Decepticon logo on the barrel and the Decep-dog on the bonnet. The toy does not have the logo (neither the Devastator toy nor the Voyager of Mixmaster/cement truck have the Decep-dog), which works for me.
What do you think?