I don't understand why folks always jump on the legality of "double towing" as every state has different laws. So why not just assume the OP has done his "legal" research and just answer the question?
In Indiana (yes, folks double tow quite often here), will re-inforce a hitch welded or bolted to the trailer frame. Of course, every trailer is different, trailer frames are different, some are more sturdy, some are more light weight.
My 2013 Keystone Outback 298RE came factory installed with a rear hitch. It was reinforced with the sewer bumper and the trailer frame. Granted, it's the smaller receiver hitch, that might be adequate to pull a light weight trailer or a cargo cage off the ground. It has a 700 pound warning label on it. I don't think I'd want to add 700 pounds, but quite obvious Keystone Outback put it there, so it must be adequate for lightweight towing.
Your frame is most likely OK too. But before you do, you might want to check with Keystone. Quite obvious your camper is now out-of-manufacturer-warranty,so you "can" do whatever you want.
I do know, in Indiana I see a lot of double tows, with 5er's and TT's as the first towed vehicle and boat behind (on both set ups). If it didn't work, or was unstable, folks wouldn't do it.
Back of my Outback. Because the bumper is strengthened with the hitch, I can attach one of these sewer bumper hitches for something like this. Look close and you can see the factory receiver in the middle under the bumper.
