Denise, we too have medical needs which means using a genny on non hook up sites (NOT hairdryer etc)
If you really do need one, then you need to look at ones with inverter technology built in which give a smooth sine wave regulated output.
They are not cheap!!
Do not go for a cheap 2 stroke as they need regular maintainance and cleaning, they are prone to large voltage fluctuations when running low on fuel and are more likely to give your caravan a large "spike" of power which will destroy any voltage sensitive equipment, such as chargers, your nebuliser, and transformers.
If you can source a Kipor 2000 Ti Sinemaster, which is a 4 stroke, it will set you back about