How much solar do I need ?

I was thinking of buying 1.6 kwh set with 1100 inverter and 400 watt panels from Amazon it's on sale for $800. I just need to run my septic (pressurized system) and well a long with water heater and some lights and regular appliances for small RV. I have no idea how much solar I actually need and it's pnw too. Also a lot of high trees so I think my sunlight hours will be less.