Please do post the wiring diagramSeems like you have a setup similar to what I had before updating a little.
It’s easy to upgrade, no need to junk anything old, just integrate the new stuff.
Best possible upgrade is batteries, especially if you don’t hook up so much. 200Ah of lithium is about £800 these days. I bought 2 x Renogy 100ah with built in bluetooth. You just download an app and check battery status via your phone.
For summer, next best upgrade is solar. The more the better. You also then need a decent charge controller.
For winter, you need a B2B charger. It’s possible to combine a decent MPPT solar charge controller and decent B2B in one unit.
To keep the old systems, you can use a 50A ideal diode to permit the lithium batteries to discharge via the Elektroblock, maintaining the function of the block and any remote gauges. The diode will not allow the onboard split charge relay to try to charge the lithium’s when engine running, thus protecting the alternator. Also prevents a circular connection from 240V, when powered by invertor, to the lithium batteries.
Connect your invertor to your lithiums, and wire it directly into your distribution board, using a 3 way switch to connect either the EHU input or the invertor (but not both) to the distribution board. Makes it fool proof.
The vehicle lead acid starter battery remains separate, and continues to be charged direct from the alternator. All old systems continue to function exactly as they should. I can send a wiring diagram if you like.