Battery dilemma

Having built myself a new 280Ah LifePo4 battery pack for our motorhome, I now have my previous 120Ah LifePo4 battery pack looking for a job.
With being mostly off-grid when away in our van over the winter months and our 410 watts of solar not doing much to help with the low sun at this time of the year, I was looking to temporarily use the now spare 120Ah battery pack to increase our battery capacity at these times.

Now my dilemma is, do I connect in the negative if the 120Ah battery pack to the load side of my shunt or to the battery side of my shunt.

Interested to hear the views as to which side of my shunt I connect to and the reasons why.

Just leave it hooked up. Parallel batteries and then the shunt. Current sharing between the packs will be proportional to their capacity.

Unfortunately, space is at a premium in our motorhome, so carting the extra battery around in the other three seasons when our solar more than covers our usage is not really an option I wanted, hence wanting the 120Ah battery as a temporary addition as and when needed.
I'm going to be cabling to the temporary battery which will be located about 5 feet away from the 280Ah leisure battery and connecting via an Anderson 175a connector for easy installation and removal.

You want the battery negatives tied together. That way your shunt will show both (but will need the new capacity number programmed in). Anderson is a good way to go. You should have a fuse on the 120Ah battery positive to protect that wiring. Be cautious making the Anderson connection, batteries need to be at a close level or the charged one will dump a bunch of current into the lower SOC one, and perhaps exceed the BMS rating.