I think the problem was the wifi at your RV park. Most likely it was not configured correctly, so having issues letting your browser certify the SSL. Or also possible it was hacked - or someone in the park created a wifi network that looked like the park's WIFI, and was trying to trick people into using it, to grab login credentials. It’s usually not hackers, it’s usually a configuration problem with the WiFi host - something as innocent as wrong time on their servers will cause ssl to fail.
I almost always use my hotspots when out of my house, well at least when I used to leave my house. Would never trust a hotel wifi. We have some users who make use of nord vpn, which is good idea too -- especially if you plan on using suspect wifi sites, in regions where hotspot isn't possible.
Our SSL cert is definitely valid. We are actually paying an extra couple bucks a month for an ssl upgrade, that is more typical for ecommerce platforms, and is probably overkill for the forum hosting. So I am mostly confident that this was not a 'server side' issue.