Well, first the 8000 W rating is probably not steady state but surge to cover things like that fridge starting up. Steady state max should be 7200 W. Think of that as a resource pool to draw from. Whenever you plug something into your panel (via your wall outlets) it uses up some of that capacity. All you have to do is limit what you turn on so it doesn't go above that 7200 W pool. When you have the fridge and freezer running, the left over 6000 W or so is available capacity that is not being used - and it will use less fuel providing 1000 W than 7200 W to reflect that.
You don't have to worry about current any more than you normally do by not trying to run a space heater and hair dryer or microwave on the same circuit at the same time, because those kinds of things are big enough by themselves to fully load a 15 A household circuit. The generator available current is not limiting you in the 120 V circuits, your 15 A panel breakers are doing that. But again, that's all exactly the same as when you're connected to municipal power. (Municipal power is a limited reservoir, too, but it's determined by your breaker service and is big enough that you can never run up against its limit. Instead of 7200 W from your genny, your municipal service will provide at least 12000 W and probably more like 24000 W.)
Realistically, the only time the lower Amperage of the generator might matter is if you have an electric stove. An electric dryer will run on 30 A, but running it will use up half your available capacity from the generator.
Long story short, you can basically keep adding 120 V loads until you reach that 7200 W limit without worrying about current.