I just did this on my 27fb last weekend. I bought the Victorn 100/30 smart solar charger and a pair of 40 amp breakers. AM Solar has a lot of info, and here is one DIY image they provide that is similar to what I did.
I did not install a cutoff switch, but instead installed a 40 amp breaker as a switch, and where the 100 amp breaker is I installed the second breaker, in the AM Solar image. Instead of going to the battery directly, I did what the Atkinson charger did, and went to the DC Bus Bars in the grey DC box.
My method was, disconnect the yellow wire from the solar panels and secure it away from everything (under my kneeling pad). Removed the Atkinson unit (including the 30 amp fuse/red-wire in the box) and remove the negative wire going to the silver bus bar with all the screws in the grey box.
Added a black wire to go from the Victorn's grounding side screw to the silver negative bus bar, add another black wire to go from the Negative out for the Victron's battery feed to the same silver/negative bus bar.
Wire the green wire that I detached from the silver bus bar (the yellow/green pair from the roof solar feed) into the Victorn's PV/Solar negative terminal.
Add a new red wire run from the copper bus bar where I removed the old Atkinson unit's red wire/fuse to the first new breaker. This will be the red switch in the AM Solar image. Turn off/trip the breaker and run a new red wire from that breaker to the positive (+) on the Victron PV/Input port. This keeps the Victron from seeing the 20+ volts from the solar panels before you set the unit to be 12v
vs it detecting and self selecting 24v. You could cut off the lug terminal of the existing red wire and use that if you wanted skipping this breaker... but your call. It is easier to flip a breaker vs pull a fuse, and breakers can be reset if they pop.
Attach the yellow wire detached from the old Atkinson unit on the other side of the Solar panel side circuit breaker. Keep the circuit breaker open or if you use a switch, open until after you are done with the battery side of the Victron, and have powered up the Victron on battery power only, no solar feed.
Pop/open the second breaker, and run a new wire from the Victron positive battery terminal to the second circuit breaker (100amp breaker in the AM Solar image). Run a new wire from the battery side breaker to the positive copper bus bar.
Double check everything, make sure cables are tight in their terminals. Reset the battery side breaker, and your Victron should light up. Connect with Bluetooth, and verify the unit is set for a 12v
system. Then reset the solar side breaker or turn on the switch if you did the AM Solar setup exactly.
Here are a few photos showing the grey DC wiring before I started, how I mounted the Victron and two breakers, and finally closed up everything with it working. You can see the green and yellow (taped together in one spot from the factory) wires snaking along the floor below the grey box, and you can see the red in-line fuse going from the Atkinson to the positive copper bus bar. I removed the Atkinson unit, left the ethernet cable (blue) in the box and removed the display in the kitchen and replaced it with a stainless steel single gang cover plate. Some day I may add an esp32 LCD unit with custom code on it with power over the ethernet cable, but not today.