Gary,
It was good to meet you in person at the show. I'm sorry we couldn't spend more time together discussing this challenge.
First, it is very, very important to remember that an FG, such as Lehel's, that has a flatbed U-bolted to the frame, is not the same as your truck. That flatbed provides a very strong and significant stiffening component to the FG frame, dramatically limiting the flex that is an inherent part of the FG's frame design.
The FG frame flexes a *lot*, even when exiting a driveway, much less over real off-road obstacles. If you have anything rigidly bolted to the frame, all the flexing forces are being taken by whatever is rigidly bolted to the frame. In your case, that is your camper box. Don & Kim built a strong camper box, and, up to now, it has been up to the task of absorbing those forces. The bending you see in your frame may be the result of those forces.
The absolute best way to mount a payload to an FG is the Ozzie method pioneered by All Terrain Warriors and later adopted by Lance's company. It's the same basic design used to mount milk and other tanks to straight trucks like your Fuso.
The advantage to the Ozzie payload mount for your FG is that it distributes the weight of the payload down the entire length (longitudinal dimension) of the frame, while allowing the frame to flex at will.
Up to the limit of its travel, the Ozzie payload mount design prevents the FGs frame flex being put into the payload. In this case, put into your camper.
Lastly, even though it is a tempered frame, it is OK to weld on the FG frame, but only within specified dimensions and places.
The information you need to do this properly is in the attached excerpted Fuso documents that detail how to properly modify an FG frame, including how to properly weld on the frame.
Excerpted Fuso FG84DE6 body builder's docs:
http://www.hackneys.com//mitsu/docs/Fuso-FG-frame-mod-pages.pdf
Please note these body builder documents are *not* for your generation Fuso, they are for ours, which is one generation newer than yours. You may be able to download the body builders documents for your generation FG from the Mitsubishi web site, which is where these pages came from. The general principles will be the same.
Doug