I had the same issue and could not get Fuel V8 running behind proxies.

I installed Fuel v9 on a Lan with direct outgoing internet access (but no incoming), then added an interface to Fuel to the internal network where Admin/PXE etc was configured. (V9 allows the PXE interface to be changed). No nodes have no direct outgoing access (so better security). This setup works fine, but has the disadvantage that fuel has a foot on the non proxies Lan.

What kind of proxy are behind? fuel only reaches internet over http. So you should not have any problem.