First off I would like to welcome you to the forum. Before you get to worried about having welded a hitch to your trailer and ruined your frame and warranty, let me tell you that I have had a receiver hitch welded to the rear of our Rockwood 8281SS for several years. We use it primarily for carrying a generator carrier, but have also used it with a bike rack. So far there have been no problems or issues with it.
There is a prevalent belief that welding anything to the frame will void the warranty of the frame. If you ask anyone without the the knowledge or authority to authorize a modification, you will get the "liability" answer that anything you do will void the warranty. This is understandable, because if they tell you "sure, go ahead" without knowing exactly what you are doing or the engineering behind the project and something goes wrong, most people will hold them (dealer, service dept, etc.) responsible and say "they said I could do it".
The Magnusson-Moss Warranty Act protects consumers from having manufacturers void warranties because of aftermarket products or modifications unless the product or modification specifically caused the problem or failure. An example is that you put aftermarket tires and wheels on your tow vehicle and use a tuner and later one of your power windows stops working. The manufacturer cannot void the warranty for the power window claim. So simply welding on the hitch does not "automatically" void the entire warranty for the frame. Of course if the frame cracks at the welds for the hitch, you are probably out of luck.
The photographs of the frame cracks are from a Lippert frame. You may or may not have a Lippert frame. I don't know which frames are used on which models and production years. Fortunately the frame for our Rockwood is a Dexter frame. If you you google "Lippert frame failures", you will be reading for awhile. If you google "Dexter frame failures", I am happy to say, not so much.
In my case, it took many, many, many telephone calls. We had to establish the capacity of my frame, my actual weights, the anticipated weight of my hitch, carrier and cargo. Next involved a reputable welder with experience in frame repair and modification working in conjunction with the Dexter engineer. Ultimately they did not "just weld" the hitch to the frame. They cut a section out and sort of "dove-tailed" the hitch into the frame and then welded on an "arm" to control the torque forces created by having the platform extend out away from the hitch. I signed an acknowledgment that the hitch is not for towing, it is for weight bearing purposes only and it has a weight rating.
I am not trying to start a debate, just sharing my experience. I do not for a moment doubt there are many which have been told "it will void your warranty". I was told that by many people (dealer, service persons, initial Dexter contact). However, unless it is someone with the knowledge, training and experience about the situation, all you are getting is the "liability answer".