Ok.... Changing to Lithiums ......
Cost of batteries - the clearest up-front cost you need to account for.
A way to monitor the
battery state of charge (SOC) - this is something that many sellers don't tell you about but is
essential as what is in your motorhome will not do this - This may be built into the batteries in some cases, or may have to buy a monitor. Costs are between £100-£200 depending on what monitor you get (but that £100-£200 will give you a top-class quality
battery monitor)
Ways to charge: -
Mains Charger. Maybe £150 if you upgrade what you have?
recommended but not essential.
Solar Charger. Between £60-£150 for a Lithium capable unit. We'll call that
essential going by what you say
Charging while Driving. Let's assume you will need to fit a B2B, so call that
essential. Say £200 for a 30A unit
Updating
solar panels? nah, definately
optional. If you have room for more and you want to be more off-grid, then yes, get more, but that would be true if you went for Lithium or stuck with Lead.
So maybe talking upto an extra £500 in parts on top of the
battery price plus any install costs if not doing it yourself.
Given your existing setup and I would guess a bit of a desire to stick with what you know (nothing wrong with that), I think I would be looking at a nice pair of
Lead Carbon Batteries. These are significantly superior to standard Lead Acid ones (with a superior price to match, but nowhere near Lithiums - I have 3 100Ah Lead Carbons and altogether they were still under the price of a single 100Ah Lithium), are much more suited to Wild Camping than basic Lead and you can take them down lower without problems. They will also work with your existing setup as well as your current batteries (actually better as more resistant to poor or part charging).
Check out this link for a good selection -
https://www.alpha-batteries.co.uk/shop/?s=lead+carbon&post_type=product . But before you do, make a note of the HEIGHT in your
battery box as well as the length and width. this is essential as many of the Lead Carbons are heigher than your typical Lead Acid batteries.