Your Jan jaunt is going to pose your biggest challenge because, you want to be able to stay put for a few days without EHU AND you'll not get a lot of
solar at that time of the year. Having said that if you only really use the leisure
battery for the bed and lighting you should be OK without adding a second
battery, the problem is if the weather gets bad and you use a lot of power using the TV/laptop because you want to stay in, if you're getting no
solar either then you'll be in trouble and having twice the
battery capacity would be a huge benefit BUT if your van has a standard charging system with a split charge relay the amount of charge current is limited somewhat by the system wiring etc.
Picture your
battery as a tank of water that you're using during the days you're parked up, because you're using water at a faster rate than you're putting in (charging) eventually the tank runs dry ( you go flat) . Simple enough
Now picture the charge current from your alternator when you're driving as your filler (charge) pipe, the alternator has the ability to pump water really quickly into your tank BUT the van wiring is acting like the pipe has a restrictor valve and the water delivery is restricted to a trickle. If driving for say 3 hours isn't enough to fill 1 tank back up to full then adding a second tank will give you no benefit because instead of having say 1 tank at 90% full you now have 2 tanks at 45% full.
Having man-splaned all of the above I'll back track and say the above is not completely accurate because electricity doesn't work like water and adding a second
battery WILL increase charge rate BUT nowhere near double it.
You'll only get double the storage capacity if you can get both batteries charged.
A final complication to consider is that lead acid batteries DON'T like being part charged and the longer they remain and the deeper they are discharged the quicker you kill them. For the reasons above adding a second
battery may lead to a shorter
battery life too.
Personally 1 lithium
battery and a
battery to
battery charger which increases charge current would probably be the route id recommend providing your
solar charger is suitable which it probably is.