First of all, the seat is very heavy. Any movement is best undertaken as a two-man job. You just might, depending on what's in the way, be able to lie it down without unplugging the seat wiring, but I'd definitely detach the vehicle battery first anyway.
Unless you have an aftermarket radio which requires a security code, the radio
shouldn't be a problem as the factory units exchange security info with the BCM. (Though I believe a code should be supplied for ultimate backup, so it just might be worth finding that).
I doubt your habitation door is connected to the central locking, so you shouldn't have to worry about ingress/egress if the central locking isn't working.
I took the precaution of removing any "secondary" power sources first (leisure battery, solar panel, etc.) and you'd probably need to do all that for leisure battery replacement anyway
So, the method I've used (acknowledgement to others on here):
Detach/Isolate solar panel.
Isolate leisure battery (switch at EBL or remove earth connection - the latter might not be an easy option at this point for the underseat battery).
Clear flooring and access to battery.
Quiesce systems and leave for 5 minutes (see below for detail) then remove vehicle battery earth
Remove the battery cover , close all doors and windows, activate the central locking with one press, key out of ignition, wait two minutes, ( wait seven minutes if the ignition has recently been on). All modules are now in standby mode, disconnect the battery negative quick release, exit via drivers door, lock with key.
Do your leisure battery work. When finished, re-attach the vehicle battery as follows:
Enter the van using the key , close drivers door, connect negative cable, wait one minute, carry on as normal. It's possible the alarm will sound, cancel with the fob.
Test ignition on
Replace floor and carpet
Re-connect leisure battery (EBL switch)
Reattach solar panel
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The following might be teaching my grandmother.....
Lithium batteries are not a drop-in replacement for AGM in all circumstances. With a decent battery management system the Gel setting on the EBL might work acceptably for a charging regime, but there are issues with wire and fuse sizing, particularly for larger capacity Lithium batteries which can accept high charging currents. Hymer group vehicles (such as the Carado) may have wiring capable of the lower-capacity Lithium replacements, but if it's a big Lithium battery, I would counsel checking the wiring and fusing arrangements, and the possibility of a B-to-B charger to protect the alternator.