You need to be aware that you only get what you pay for!!
The cheap gennies tend to be 2 stroke, and require regular servicing to maintain their performance.
Also, the cheap ones tend not to have a smoothing circuit, so if the motor revs highly at any time, like when its just about to run out of fuel, you will get a voltage "Spike" which may well damage the more sensitive electronics in your van, such as battery charger.
Another point is they , all of them, are relatively expensive to run, with fuel at almost a