I found an amazingly easy fix. I am surprised Victron does not put this out on their site.
Here it is for a Multiplus II:
Go to Victron app on phone.
Go to settings
Touch Upper corner where it say ON
Drop down- say CHARGER ONLY.
DONE!
The way the Victron is designed, it goes thru the inverter when it does not need to, but Victron designed this way for other configuration as I understand it. By turning off inverter, it goes and charges or powers what you want cleanly bypassing the inverter.
An inverter is not needed to charge the battery, only invert 12v into 120v. So the PowerBoost cannot sync with the Victron inverter and gives the error. The PowerBoost only need to go thru the converter side 120v to 12v.
Also, while charging the 120v stays powering as it bypasses the inverter and lets the 120 thru.
Air conditioner and lithium battery, ebikes, all run and charge great from RV outlets.
Been on a camping trip for 2 months and works great.
Now enjoy the real value of PowerBoost!