I'm looking to get a good outdoor (weatherproof) omni-directional WiFi antenna for my RV. I'll either be inside the RV, or not very far away from it.

MT doesn't seem to sell something like this themselves.

Does anybody have a good WiFi antenna they can recommend for this application?

Explain in more detail, what exactly are your expectations from a "good outdoor (weatherproof) omni-directional WiFi antenna" that you will most likely be using indoors? And what frequencies are you planning on using?
https://mikrotik.com/product/ACOMNIRPSMA

For installation in cars, microtik makes a series of devices LtAP

Use Ubiquiti AirMAX Omni antennas with some BaseBox/NetBox/NetMetal.

On my signature you can find information about my network and you can see this combination using 2,4 and 5 GHz.

Regards.

not sure if its the "best" but the one that comes with the NetMetal 52ac works great for our RV https://mikrotik.com/product/RBMetalG-52SHPacn