Hi, the company I used and can highly recommend is RoadPro in Daventry. Our van came with the additional AGM battery, even with that and 20O watts of solar it was to be honest rubbish, ok if you plan to use EHU but after two days with no solar input battery was failing.
Our van has a compressor fridge and diesel heating (both brilliant) which can be heavy on battery power.
RoadPro are not cheap, a small company with an excellent reputation.
They fitted two 100 Ah lion batteries, a small NDS inverter, modified wiring and fitted a Votronic control panel.
Cost was just under 3k, but I believe battery costs have fallen since I purchased, so would imagine a cost of 2k with no inverter.
Don't be fooled in to just dropping in lithium as a straight replacement Hymer use a different set up to most, from what I can remember my vehicle came with three separate battery chargers and did need considerable modification.
In total 8 hrs of labour, including fitting inverter.
The end result transformed how we use the van, our last night tonight in the Lakes and battery still showing 26 %, five days, four nights camping.
I did run the engine for 10/15 minutes when refilling with water which replenished with additional 5% charge. Major advantage of lithium is rapid recharge when running the engine compared to AGM or similar.
As for solar panels, Campersales had mine fitted by a sublet contractor, work fine but next time I would use Roadpro which I suspect are better quality.
One item I will fit in the future is a
Ablemail AMT12-2 Trickle Charger battery monitor to supply and maintain the starter battery.
One if the few battery masters that can work with lithium and AGM starter battery. Not available when I installed.
Most Sprinters with the latest tech struggle to maintain starter battery charge for much more than two wks if not in use, tech never really shuts down, things like electronic handbrake constantly monitoring.
You may have a wait with Roadpro, and they will only answer your initial request by email, this is basically to stop them answering technical question form free loaders. As a small company they only have time to assist there own customers.
They do provide a loan car/ or allow you to park over night
Hope this helps.