Congrats on your new Bighorn, hopefully all things will be good going forward following your warrantee work. Unless there was a dimensional change from our 2017 model to your 2018 model, the movable shelves are 29 3/4"w x 16 1/2" deep. You should be able to order an extra shelf from Heartland or as we did, I just used 1/2" thick shelf board to make our third shelf.
A note about the factory shelf system; as you know or will find out, camper walls are mostly just thin lauan with wall covering glued on, in some specific areas they do insert wood blocking but I did not find any in the pantry walls. We moved into our camper fulltime in May 2017, within a week the OEM shelf system failed due to the screws pulling out of the wall. I removed the OEM components, went to Menards and purchased 4 metal shelf tracks long enough to go from the floor and almost to the fixed shelf, this way the screws just stabilized the brackets, with the shelf weight being transmitted directly to the floor. I mounted these brackets on the sides rather than behind the shelves as they would normally be used so again shelf weight was directly down to the floor.
We use the top two factory selves for can & boxed goods and the bottom shelf carries our printer/scanner, after just under 2 years on the road this system is still working fine.
Good luck and Happy Camping!
