I always suggest professional advice. It is not reasonable to expect proper technical solutions/advice without someone knowing exactly what you have done, how you did it, and how you plan to load your vehicle. Loading behind the rear axle completely changes the stresses within the frame. Fatigue will be the killer of a truck frame. I am not sure who on this site can model that effectively.The only input I can give is I have a liftgate on my truck. It's mounting extends 2 feet aft of the Fuso's frame and it is rated at 2000 pounds.
I've had close to that on it with no problems BUT that is STATIC. Off course a dynamic load is going to put much more stress on your frame.
You might see if there is some information in the Fuso's body builder manual HERE
See chapters ( if using this one ) 1-2-2, 1-3-5, 1-4-1, 1-10-1 (I guess the "do not install here" means outside of the rail, behind the wheel because of debris).
It looks like from the 2nd manual ( BBManual 2 ) that the FE had a rear center fuel tank as 'stock', but certainly not a large as yours. Page 11-12-4
The 2nd manual should be helpful as it gives you all of the factory frame dimensions, so perhaps your 'mod' falls into one of those ranges.
It also has all the frame modulus information which is very useful to calculate the allowable loads (for people with bigger brains then me)
Without seeing the build in detail the only thing I can deduct is youll have near on 1/2 ton of load behind the axle (which is a great pivot point working against the engine and gearbox that is reasonably well proportioned over the front axle with nothing in the middle except for the chassis and sub frame of the camper bodyOk this thread has really got me nervous, I've just got done extending my frame and building a very large fuel tank (110gl). I asked a bunch of frame guys in my area and they said I would be fine as it would counter the engine weight over the front axle, but now I'm thinking I'm going to "bow" the frame? What do you guys think?