$7k? Unbelievable. Sounds like top shelf components installed by a pedi'greed' #vanlife installer. Let me guess, they can't fit you in for 6 months too?
You can handle this, and for much cheaper. I'm on my second lithium system using batteries from Amazon. My recent van build got a 300ah LiTime which is a fantastic performer so far. It was about $1k. The thousands of positive reviews talked me in to it. I'm currently finishing a 2nd lithium setup in my work rig, 560ah of Chins brand, also from Amazon. I went with 2 280ah because they were so cheap, $509 right now! I paid much more and still thought it was cheap, about $750 per IIRC.
All you need is to upgrade your battery and switch from an isolator between your chassis and van batteries to a DC to DC charger. You can probably skip the solar all together. Solar installs get expensive quick.
I have been very happy with the Kisae DCtoDC that was recommended to me by the electrical gurus here.
- Do you need an inverter? If not just get a simpler lithium AC charger (Noco or the like) and keep it topped up at home.
- How long do you need power when parked? This will entail a basic homework assignment of adding up your power needs. Not really hard to do, especially with so much info out there on mobile living nowadays. Avoid microwave/hair dryer/AC electric needs while camping and your battery will go a lot longer.
Assuming your present setup will allow a battery this size (It's lithium so best mounted inside the van) and that your wiring doesn't need to all be upgraded, though it's not expensive if it does, here is you a mostly complete system for less than a grand. Add some good quality DC breakers and get fancy with a shunt/monitoring system if you prefer and you're still out less than $500 more.
https://www.amazon.com/gp/product/B0...e?ie=UTF8&th=1
https://www.amazon.com/KISAE-Technol...e%2C134&sr=1-8
https://www.amazon.com/NOCO-GENIUS10...zcF9hdGY&psc=1