If it was me using a lead-acid aux I'd fit the following (assuming the solar panel voltage is compatible with the Victron unit):
Victron 75/15 (Smart Solar has inbuilt Bluetooth so you can see what its doing on your iPhone).https://www.victronenergy.com/solar-cha ... #downloads
Projecta 150.https://www.projecta.com.au/dual-batter ... c-isolator
150A fuse at the aux battery (doesn't have to be this brand).https://www.supercheapauto.com.au/p/pro ... 56602.html
Others will have other ideas but this is what I'd do (and have done on my car). The only difference on my Hilux is I put the aux battery in the engine bay using one of these brackets at the end of the post (mine is stainless though). The reason is this allows you to avoid running a long pair of very large cables to the back, they are very short when the battery is in the engine bay. Then I just ran a figure 8 double insulated about 8-10sqmm to the rear through the chassis rail to the solar regulator mounted right next to the fridge. The solar panel sits between the roof rack rails on the canopy and feeds straight down to the regulator. In your case you'd just run the solar infeed to an Anderson plug and plug your solar into it. You can run smaller cables from the Projecta unit to a battery in the rear if you like but you won't get the added benefit of being able to start your car using the aux battery if the main goes down.
Note the Projecta I just mounted using the bracket supplied right in front of the starting battery. The bracket simply gets held in place by the main battery. The 150A fuse I mounted up on the vertical side piece of the aux battery tray. 0V lead from aux battery goes straight to the 0V post on the inner guard where the starting battery 0v lead goes to. 12V lead from the aux goes straight to the 150A fuse, and then a short lead to the Projecta, and the other side of the Projecta has a short lead to the starting battery 12V post. I also mounted a 35A fuse carrier and fuse on the side of the aux battery tray and then ran the figure 8 to the solar regulator in the back.https://www.ebay.com.au/itm/DUAL-BATTER ... rk:27:pf:0