Finally buying a strong inverter. How many batteries to buy?

I do know one thing for sure and that is my next TC (if I buy another one) will not have a forced air furnace. It will have a Wave heater than consumes no electricity at all. A Wvae heater and a casette toilet and no black tank to deal with and probably a Danfoss Compressor fridge too. Between the LED lighting (which I already have, every light, inside and out is LED) and the other items, my electricity needs will be very low.