While my experience applies only to my 30' Classic it appears from the comments on this thread that it is similar and I made be able to shed some light on what is and how to take care of it.
Simply put Airstream design engineers did not complete the task of making it possible to use any portable satellite receiver with their coaches over a number of years.
Such was not the case with my 2004 Classic. Reach inside the credenza to move a coax cable and you have a satellite connection.
With my 2018 Classic as previously mentioned "The manual says "Your unit is pre-wired for a portable satellite antenna....The interior pre-wire, consisting of a 2-pin power/ground connector and a coax cable pigtail, is located in the dinette roof locker under the Blu-Ray Player."
This is misleading at best and potentially expensive to a new owner. To pay in excess of $100k for an Airstream it seems a poor decision on Airstream's part to stop short of completing this task.
So I've done some engineering as follows:
Materials needed: 1 passive HDMI splitter and 1 passive HDMI switch. By passive I mean no external power is required. All parts are 4K 3D HDMI 2.0 certified parts. I also include a couple of 3' HDMI cables and a few zip ties to clean things up. Total cost, under $35.
I purchased from Amazon two HDMI devices. One a passive switch the other a passive 1x2 splitter. https://www.amazon.com/gp/r.html?C=2...I8KNGZ3G4HAZEA
AND https://www.amazon.com/gp/r.html?C=2KVQPKQSS2V7N&M=urn:rtn:msg:2018072923201 2979748ca03c944fb83e490827eb0p0na&R=1SXULQ96VZD9X& T=C&U=https%3A%2F%2Fwww.amazon.com%2FMonoprice-Blackbird-Compliant-Connectors-Bandwidth%2Fdp%2FB01CT8GHE8%2Fref%3Dcm_sw_em_r_dp_ dz_U_5TKxBb76652B4_tt&H=E7TGIAD9ASJMBU5XMMXX4LCAFP WA
Remove the cabinet or part thereof that contains the Blu Ray player. It will be held in place with 2 or more screws hidden by screw covers along the bottom edge.
Behind this cabinet you should find a bundle of wires of various types including among them a black coax cable. This is your satellite connection to the outside. Clear this cable out of the bundle and move it to the nearest sidewall. I then rearranged the remaining wires to a neater bundle and zip tied them together in such a way that the inline fuse holder is easier to get to. Still don't know what the 2 pin 12V
power is for.
Start with the passive splitter. Remove the powered splitter installed by Airstream which frees up a 110V socket for your satellite receiver. Replace it with the new splitter. Connect the HDMI input cable to the passive switch.
I chose to mount the switch on the outside of the Blu Ray cabinet. So I used a drill to create two grooves in the bottom of the cabinet to pass through the HDMI cables to and from the switch. One from the splitter and one from the Blu Ray player. You can mount the switch on the cabinet with Velcro or the like.
Taking care to arrange the many wires insure the coax cable and the Blu Ray power cord are in the back corner and not pinched, reinstall the Blu Ray cabinet in the overhead cabinet. In my case, above the dinette.
You will now be able to install your satellite receiver of choice. You will need to include an additional HDMI cable to connect the receiver to the HDMI switch. When you turn on a the TV and the satellite receiver a light will illuminate on the switch. This indicates which side is feeding the splitter and thus the TV's. Push the button to change sides.
If this is your first time connecting a satellite antenna you will now have to follow the satellite antenna's connection procedure as outlined in it's owners manual.
Hope this helps.