Multiple networks in wpa_supplicant.conf

I am using HA in my camper van. There is a local wifi (router) it connects to. The router connects to my house wifi when it is close (and to other AP’s as I travel about). I would like to have the HA switch networks (e.g. the house & van) when the router is down or I have taken the electronics module into my office to work on.

The problem is that I cannot seem to get HA to switch from one network to another after boot. Whichever network it connects to seems to be the ONLY one it will connect to until I go in via ethernet or (re)boot with a monitor connected and manually switch.

I created a wpa_supplicant.conf with both networks listed. Shouldn’t the underlying OS pick the strongest signal and connect to that?

A side issue: why doesn’t the HDMI work unless a monitor is plugged into the RPi at boot time? Another config detail I need to address?

TIA and I apologize if this has been discussed and solved - I looked around a bit and didn’t find any obvious answers. A pointer to the answer would be appreciated if I missed it.