Adding a rear hitch...

The welding shop should be able to talk you through if you need to beef up the frame.

Ironically, I've found that most welding shops are pretty poor at figuring out where structures need to be beefed up.

I put a hitch on the back of my 5er. If the hitch weight of the towed trailer is low, I don't think it affects the TV ride that much. I've never found it to be an issue.

I think towing doubles is fine for light trailers, less than 2,000 pounds. Once it gets above that, I worry that the 5er frame isn't up to the forces that a larger trailer could put on it.

Whatever you do, stay under the weight rating of your pin box and truck hitch.

If the second trailer is sufficiently heavy, you'll need to wire a brake feed for it.

I have towed small boats behind my 5er. You literally don't know they are back there. A backup camera on the back of the 5er is an excellent idea so you can actually see what is going on.

I'm typically over length, but nobody has stopped me or complained. I get lots of gawks though.

Not being able to back up is a huge pain, particularly when getting fuel.

If your 5er is long, you really have to watch how high and low the hitch on the back of it is going to be when going in and out of various places. It can make for very dramatic tilt angles on the 2nd trailer if the hitch to wheel distance is short.