@ShyTRex - here's an image I had saved to Pinterest long time ago that may be of some help in the way his main center slide works.
No long drawer slides, no roller bearings, nothing but wood. The side boards for the middle tray are captured by the trim for the side cabinets and act as counter support when the tray is cantilevered out. If you were to do this with a simple tray in your situation, you'd want a removable stop so it doesn't pull out any further than 4 or 4.5 feet, leaving enough to catch against the trim for the side cabinets or whatever you put in place.
I've also read about contractors who have made such things and had them slide only on hard plastic washers, sometimes plastic runners. Full size trays loaded with carpenter's tools, paint, table saw and such, and worked famously well. The right combination of polycrylic-ed surface and plastic washers or runners (like is used for table saw sleds) and you could get out of this very inexpensively.
If you make the stops removable, then you can take the whole tray out when needed to use the trailer for other things.
I'm seriously thinking of experimenting with this on my trailer, which is almost eight feet inside (pic at bottom). It could make loading up and then grabbing what I need far easier than climbing up in there with/for every item.
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