Okay I guess i'll do this more in depth.
Please keep in mind my interest is on the MOTORCYCLE not the trailer, i'm solely interested in how to put a hitch on when literally every motorcycle manufacturer I talked to says its not recommended yet people clearly do it.
If building my own trailer I will probably use or start with one of those "CarGo" type rooftop carriers on a frame for the majority (not all) of use. They start already streamlined, are light, are lockable, and do the job. I'd have it removable for if I need to haul something bigger or less aerodynamic - i'd actually like to extend the frame or something to haul a couple of sheet goods (4x8 feet) on top of it. Not superheavy, and not fast - more like things like 4x8 sheets of 4 inch thick styrofoam insulation taken out of freezers for other projects, which are bad enough aero stacked up I wouldn't exceed 35mph anyways. Or the occasional couple sheets of plywood for a project when it doesn't justify using the minivan or car trailer. The latter would be unlikely to exceed 150lbs normally, more than 3 sheets and i'm likely to borrow the minivan.
How much groceries at once? Well technically alot more than groceries thats just a figure of speech, I buy and reuse alot of stuff so pick up all kinds of things from Rock Band drumkits to repair and resell (which are real bulky but light) to scrap windows i'm saving for a greenhouse project to anything else imaginable including weightlifting plates when cheap, and I normally pick up multiple things on a single trip to save more gas - better one zigzagging 4 hour trip around town and a full trailer, than going all the way out and back home one load at a time. It's fully possible that a geo metro with a little 4x6 trailer will do a better job but I want to research all possibilities first plus I kinda want a bike anyway just for when i'm not hauling the trailer.
My max load goal is 500lbs - Even 500lbs of trailer will be less than me + the bike and even 500lbs at 35mph should be less brake load than unladen at 70mph - the sole questions are balance and a properly mounted hitch that wont tear the frame when you hit the brakes or unbalance the bike unnecessarily. The request for highway speed as an option would not be combined with a "full weight" of up to 500lbs ever, it would be one or the other - it's just there are legs of traveling around the twin cities in MN that REQUIRE you to be on the highway for a brief period - so the ability to pull up to 55mph for up to a few miles to cross one of our stupid rivers before I can pull back into town is vastly preferred. Sometimes there is a "through town" way to do it that's 10 miles out of the way, and since that's crossed nearly every time I go shopping that kills a few extra mpg savings.
My problem is I cant even search stupid google for "motorcycle trailer" without getting 100% trailers that haul motorcycles, rather than trailers hauled BY motorcycles... there is zero way to differentiate to the search engine what I want and mean so I can't even find if there are good forums for this. Plus most people haul with big stupid 900cc cruisers too but there's no point to a 900lb motorcycle getting 35mpg, my sole focus is on efficiency of moving the loads. If a 125cc can do it (they can reach 55-60mph on flat roads normally) around town on the flat i'm happy, if that fails 250cc is fine too. Much bigger than that and I fear a Geo Metro makes more sense because the "huge mileage benefit" rapidly slides. I might even rig up some kind of electric or surge brake for the trailer, we'll see.
It sounds stupid but i'd actually like to stick a hitch on a Ninja 250 if possible - i'm aware that is a sportbike, but it's what people are getting 100mpg with here. I don't know why it should be any stronger or weaker framewise than a 250cc standard like a Rebel or something so I figure why not. The only previous hitch on a standard i've seen up close involved four attachment points to pieces of frame, I don't know if thats standard or not. And a "500 pound trailer" I don't think is insane when i've seen 500lbs moved on mormon handcarts and by bicycles on flat ground, hell I saw two 300lb fat people on a single Ninja 250 that amazingly wasnt complaining and they rode it all day every day with both of them two up forever. What i'm doing is "not what it's designed for" but I think there are worse abuses - the linear loads should not be obscene, it's more likely a question of how the bike handles when it's say stopping and turning at the same time. Which at low speeds is less of a terror.
Worst case if it's too squirrely with 500lbs (which riding it would prove as I test it with added weight plates) I make a point to lower it to 350, or 250 or something unless i'm going 20mph ultra locally. I just want to know how I design or make a hitch that will put the load in the best place it can be, spreading it out for instance, and not upsetting it more than it has to, and mostly to not rip the frame of the bike going over a bump or pothole. :P The sole reason for "pushing limits" is the more I can do on the motorcycle the better the fuel economy vs having to do it with a car or small pickup. It's just i've heard 500lbs trailer on a decent sized standard shouldn't be that big of a deal - it fits within the classic "dont tow more than your tow vehicle weighs" limit, and though a 125/250 is maybe 90lbs lighter than a 650 they still have to have brakes to haul themself down from highway speeds and stuff just as competently. Even 300lbs towing is useful.
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PS to weight transfer on braking, as long as the force were transmitted to the rear of the bike, i'd wonder if it might even brake BETTER since the toungue weight of the trailer would counter the nosediving of the bike under hard braking.