Are you running a 12v setup on your camper for Starlink? We have the same issue and like you, it started out of the blue earlier this summer. I have tried 2 completely different 12v kits and they both did the same way.
As stupid as it is, I added a 12v industrial switch in between the 12v setup and the MaxBR1 Pro 5G and my speeds are better. Still not as fast as using the SL router and connecting to its router directly, but at least its usable.
It worked all Spring and into the summer and then all of a sudden speeds were crap. Nothing in my hardware setup changed, so it has to be something on the SL dish firmware or the Peplink itself.
In my 12v setup, I do not use the SL ethernet adapter, so it cant be blamed for this compatibility issue.