Bill, I can't help you anymore except by bring to you electric theroy that I am an expert on as far as this forum is concerned. I have been doing it more than 60 years if that means anything.
Rather than get into a debate about it I will not answer again but leave you with this common sense.
A fuse, any fuse absolutely does not blow unless there is to much current flow through it. If the fuse does not blow until you plug a trailer connector in, it has to be the trailer causing the excess current to flow.
Again having anything else on has no 'electrical' connection to any other circuits (In fault free wireing) . Why would it?
Even after plugin without blowing, the first time you use the turn signal it should blow it again but you say no. How come?
There may be a relay involved in the distrubition box as well that may have some type of fault.
I added trailer harness, fuses and relay to my truck with both 4 pin and 7 pin outlets plus electric brake controller and have never had any type of reaction like you have.
I have towed large trailers with electric brakes and still tow small trailers with 4 pin trailer lights.
It's not a Ford fault in design.
You have a specfic fault to your situation and have to run the reason down.
Good luck with it.

 

Just bought a 1999 f-250. Gently easing it back to normal. One niggle has popped up. Right rear tail light brake at first was intermittent along with the high mount brake lamp. Now it will not show brake. It will show running light, it will not show signal. The bulb is good. The socket is good Disconnecting at the connector near the left wheel and testing with a probe light I find blinking on only one wire. The power is not coming from the front of the truck to feed right side. The fire wire at the right rear is orange with a blue tracer. Next: Probing under the hood under the air intake I find an orange wire with a blue trace. Test light produces flash when the right signal is on. Therefore orange-blue wire is hot under the hood but not at the connector. Is there anywhere to look for a wiring diagram for the drop nosed f-150/250

 

There is a harness plugup toward the rear.
This is were the harness plug is for the wire option of trailer with 7 pin socket or no 7 pin socket.
Follow it toward the front from the driver side rear till you come to the plug and test there for an issue.
Should be somewhere over the wheel well area inside the frame.
Good luck.