Auxiliary fuel tank in front of the slide in camper

Technically, it sounds like it can be done, although it will look a bit "off". The 815 is a fairly lightweight camper that can be carried safely on a short bed, AFAIK. And your long bed dually truck should have no problems to carry it that far back.
You might set yourself up for a catastrophic outcome if you ever get rear-ended in that setup, though.
Other people with more experience with aux tanks may chime in with a more reasoned opinion.