I'm sure it is safe enough although it would be a little unusual looking. It would be better to have the extra fuel lower and under the bed, and tanks are available for that. You can mount batteries either in the corners of the bed or on the frame as well. You could also add solar or a generator to use less batteries.
It is true that with respect to the rear tire the weight will be in the same place, but the relative distance to the front wheels would be 1 ft further back which would impact handling.
I'm curious though ... what is special about the 5 1/2 camper you are looking at?
Kayteg1's example isn't quite the same since he moved the end of the camper is further back compared to the rear wheel. It's got its own set of potential issues. He doesn't have a 9 ft bed in other words.
