I use a SOHO Mk3 in my RV and have a Panorama 5 in 1 external antenna I use with my AC791L for LTE but it also has a 2x MIMO for wifi as well.
I tried various combinations with the SOHO 2 internal and 1 roof connected, 1 internal 2 external, 2 external 3rd empty.
I am currently just using the 3 internal antennas that came with the SOHO and not the external at all for wifi. My RV is fiberglass and it gets a good signal inside and out either way and I want to maximize internal performance anyway. Connecting the external antenna slowed down internal slightly while not making much difference on external in the places I have tried. Most places have horrible wifi anyway so I just use cellular for wan.
Be sure to disable WiFi wan if you move the RV as the SOHO has a big issue that if its enabled and no access point is in range it will rescan every few seconds for it which disrupts LAN wifi while it does the scan causing packet loss.