A simple test is to install the modem direct to the cable and do a speed test (speedtest.net) and then do the same connected to the inside.
I used to install cable and found poorly made ends that did not all signal through, loose connections, corrosion, bad splitters, screws and nails running through the cable, etc.
Takes some time to figure it out. If a local cable guy comes into your RV and fixes it give him a nice tip as this is not his responsibility plus coffee and a donut!
2017 SLX 195RB
2021 Jeep Grand Cherokee Summit L 5.7L V8
Andersen WDH hitch, Renogy 100 AH Lithium &
200 Watts solar panels from Renogy
Prev. '14 Jeep Grand Cherokee Overland, gas 3.6 V6