Do you actually need an inverter or can you manage with 12v only when you are not on EHU? Even with an inverter you won't be able to use high powered appliances like heaters or kettles because they will take too much out of the batteries.
Chargers for phones, eReaders, tablets and laptops can all be run direct from 12v. The only thing we need an inverter for (and that's only a small one) is to charge camera batteries for Canon cameras because they only supply mains chargers. Even many TVs (and satellite boxes) are available that run directly off 12v rather than suffer the overhead of an inverter.
So, it's worth working out what you will actually use before spending your cash
Graham