Boy... lemme tell you, I double trailed for 8 years and had some incredibly scary near wrecks from sway getting out of control. Any sway in the first trailer is amplified to the second. I never needed a camera because on even a straight road with no wind there is just enough sway to see the sides of the 2nd trailer moving side to side. These were well balanced too.
You can get a wireless camera system... put the camera on the back of the first trailer, wire power to it somehow (for short tows, you can even hook it to a 12v 5AH battery that costs $10. Mount will vary depending on if there is a back window or whatever.
Utah is the only state in the USA atm that still allows "ball to ball" towing. A 2nd trailer behind a 5th wheel is generally safe, but Utah is the last one to allow a bumper ball trailer to tow another bumper ball trailer. It is dangerous, but if you take care... its doable, i did it for 8 years like I said.
We have a Toyhauler now, I absolutely love it. My wife got so scared of double trailering she didnt care what the toyhauler cost, she was DONE with the white fingernails. We had a mishap with the first trailer right around that time and it got totaled out... State Farm treated us WAY better than we deserved, we rolled that payout into the new trailer (really wasn't all that expensive all things considered).
So bottom line, never needed the camera when we did it, but there are wireless kits. Some need a signal booster if the distance isnt too great. Be **** careful though... be warned, every trip could be a rodeo you really dont want to be in.
-DallanC