Are you referring to the LATCH child seat system?
There was an odd bit of information rolling around one of the seldom used portions of my brain left over from a Safety Systems Class I took.
I did a quick search and confirmed the LATCH systems aren’t required for vehicles above 8500 GVWR.
Here’s a link to the Sprinter Roadtrek site with more info.
No LATCH System in RVs? - Sprinter-Forum
I’m not familiar with the exact seating configuration of your Interstate however I would “Read, Understand and Follow!” the child seat manufactures instructions on placement without a LATCH.
FYI our techs are told to not mess with child seats, and, if they have to remove them for any reason the seats should be put back in the car in such a manor that no kid could ever be put back in it. Upside down on the back seat is good. The liability issues are staggering.
Many Dealerships have a Certified Car Seat Safety Specialist. If you still have questions I’d check with the Airstream or a Sprinter dealer to see if anyone is properly trained to install a child seat in the rig.