Forget that "somewhat"... that's just rare
Many of us are familiar with the Sunrader and similar small Class C motorhomes built on small truck chassis, but I have never seen a tow body like this before. It looks pretty slick. Of course the factory availability of longer cabs, longer wheelbase, and larger stock tires have made this sort of custom setup obsolete, but it was an interesting solution at the time.
The box sides look really low, but that's not a custom feature... small two-wheel-drive trucks were just that low back then.
I'm pretty sure that Toyota never provided or approved the dual wheel setup. I've seen motorhomes with different wheels on the front and rear axles, because the stock Toyota setup is on the front, and some high offset (to work as duals) wheels with an oddball bolt pattern on the rear to work with the adapters that the RV maker stuck on. Unfortunately, the dual wheels don't increase the axle or suspension strength or capacity at all, so many of these motorhomes are overloaded even when they're empty - this would be much better that way, although the engine may still not be up to the task.
In this case, the frame must have been substantially stretched, presumably by cutting it and splicing in extensions. The stretch length would equal the length of the cab extension. It looks like the stock box with fender flares added, and the stock cab plus an extension.
I don't recognize the trailer, but it's not fiberglass.