There are literally thousands of posts about bike racks here, since it's discussed constantly.
That said, I would never put my bikes on the rear of my trailer and where I can't see them. But my bikes are more expensive than most people's.
Normally I just put them in the Truck bed, under the bed cover. If I can't do that, I have a 2 bike Swagman folding rack that mounts to the rear step bumper ball. It holds the bikes high enough to clear the propane tanks, when turning. If I couldn't do that, I throw some old blankets over the bikes and put them in the trailer.
If I decided to go with something on the A-frame, I'd go with the Stromberg Carlson Bike Bunk:
Dan-Retired California Firefighter/EMT
and Zoe the Wonder Dog(R.I.P.)
2016 PrimeTime TracerAIR 255, pushing a 2014 Ford F150 SCREW XTR 4x4 3.5 Ecoboost w/Max Tow Package
4pt Equal-i-zer WDH and 1828lbs of payload capacity