Lights work, brake lights work, left signal works, but when I turn on the right signal, it blinks once, and then blinker in the truck stays solid, and the left blinker on the trailer starts blinking. What the He!!? Any ideas?
check the ground, make sure both plugs are clean and free of corrosion 
could be a bad flasher relay in the vehicle also.
Yep JDEAN is right…98% of the time it’s a ground problem (drove 18 wheeeler for 20+..have run into this weird stuff before 
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It could be the flasher… but with one side working ok….. but it’s a cheap part would not hurt to try..
Good luck guy
I am leaning toward the grounding issue myself, as I have tried on a different truck, with a different adapter. All signs seem to be pointing to something with the wiring on the boat. Having said that where do you think the grounding issue is. There is 3 places on the trailer that is grounded. At the tounge, and at each light. Just looking at them, the connections look ok, but I will take apart and clean, and see what happens. Thanks for the advise.
Also;
make sure you do not have any burnt out bulbs.
A burnt out bulb will make a blinker light on a dash stay on.
Just do what most SW Wisconsin farmers do and don’t even hook up the lights on your trailer. I used to travel extensively throughout Wisconsin and I can tell you that Tuesdays and Thursdays are sale barn days in SW Wisconsin and farmers will hook up their stock trailers and not bother to hook up the lights. This holds true for Grant, Crawford, Iowa, LaFayette and Green Counties but then in that part of the state they don’t use their turn signals anyway. I always wondered if these people actually know what that little lever is on the side of the steering column. 
Eyehunter
Don’t rely on the tounge ground only…use the grounding wire in the plug…
Use a CLEAN ground from a spot on the truck frame and make sure the connections are clean..
Otherwise eyehunters observations are spot on 
Pay attention to this This is a very common problem Take your bulbs out and check them! also the grounds are a problem I’d lean towards a bulb. sometimes the elements burn and cross in the bulb.
Electrician
One more thing to check with the flasher, make sure its a heavy duty flasher, seen this before. If one side (the right) has more resistance, it will flash on the left side and not the other.
big g 
Also might check the bulb types. I once replaced an out blinker with the spare uber-high efficiency light that I had as spare from the wife’s car. It didn’t drain the flasher fast enough, and it would blink about once every 15 seconds.
Has this always been a problem, or did this just start recently?
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