Hi Calamity,
As someone else had mentioned, the 3500 pound hitch you have on your motor home today might be limited to that weight because the chassis frame or frame extension could be limited to 3500 pounds.
In general, Sprinters made since 2009 (maybe a year or two earlier) are equipped with a 5000 pound hitch so hopefully any hitch company could simply unbolt the 3500 pound hitch and bolt on a 5000 pound hitch.
One thing certain about Sprinters and Transits. Frame extensions and welding & drilling of any kind is not permitted. RV manufactures are limited to work with what it is and they must use the existing holes for bolting a hitch. The reason is they use an exceptionally high strength but very thin steel that the heat from welding weakens, and the drilling of holes compromises. They shape the steel into a box frame to further get the strength needed.
If your 24 foot long 2012 Freedom Elite is built on a Ford E350 or E450, or it is built on a Chevy/GM 3500 or 4500, I think the right outfitter can help you. Rust can interfere and sometimes nix the project, but your rig is only 5 years old so I doubt that would be a factor.
One thing you will want to ask about is the addition of a rear trac bar, and replacing your stock rear stabilizer bar with a heavy duty version to stabilize your rear end while towing a multi-horse trailer. Most 24 foot motor homes are built on a standard 158" wheel base which means your motor home has significant rear overhang, the amount of house that hangs beyond the rear axle. The cantilever condition while towing a horse trailer should get those two upgrades for safe handling.
I would call Champion Frame Align in Elgin IL below. I endorse them because they did wonders to the suspension of our motor home. Actually both the motor homes we've owned during our lifetime. They upgrade suspensions on everything from class C motor homes to class A's to school buses, Greyhound buses, ambulances, fire trucks, tractor trailers, just about anything with 4 or more wheels. They really know their stuff and are well equipped to handle anything. I believe they do hitches too.
I know you live far from Elgin but it is surely worth a call. Maybe they can recommend someone much closer to you.
Champion Frame Align
864 Bluff City Blvd.
Elgin, IL 60120-9519
847-742-9294