A 3000 should be able to run an A/C but don't expect too much more than that. Keep in mind the number typically represents the peak wattage, not the running wattage. This means at start up it can support 3000 watts but then can only support 2600 to 2700 after that. If an A/C is running and it is taking up most of the power, then you might have to shut it off when you want to run other high electrical stuff, like a hair dryer, microwave, or a toaster.
I would shoot for 3500 to 4000 watts.
A popular, expensive method that is highly portable is two 2000W Honda generators, that can be connected together and provide 4000W or (30 amps) of RV power.
The other thing you want be be concerned with is how clean the power that is provided to the RV. Many generators provide 110v but computers, and other sensitive digital equipment need very clean power. This is another advantage to Honda, Yamaha and other higher priced generators.
I am not familiar with Lifan and I am certainly not discounting it, but I'm trying to provide what you really want to look for in a generator.