The base of the Demco Recon Gooseneck-to-5th Wheel Trailer Hitch Adapter - Single Jaw - 21,000 lbs # DM8550045 you were looking at is 30" wide and 28" long, and 7-1/2" tall as you can see in the included diagram.
The opening is 3-1/4" square and will only work with the Demco Recon Head Unit. You will not be able to use it with a square gooseneck ball as that is not what it was designed or tested for, and attempting to do would void the warranty of the adapter.
I included a video of the... view full answer...
For an RV fireplace that has fake logs in it like you mentioned and would fit your cut out well we have the Furrion RV fireplace # FF34SW15ABL which fits your cut out dimensions perfectly. view full answer...