Definatelly a diesel. The bigger the better. But alternative/backup generator could and should be petrol/gas version so if you are in the middle of nowhere you dont end up problems if the main generator breaks. There are generators that run on 1500 rpm. They are really durable and low noise but the size is a lot bigger and price higher multiple times compared to standard 3000 rpm generators. Diesel generator can produce 1 kW around 0,40 €, gas generator around 0,80 € and petrol 1,40 €. So electricity made by diesel generator is not that much more expensive than from the shore power plug where available. Here in Finland the price varies place to place but ABC Matkaparkki charges 5 €/day for 16 A 230 V shore power for RVs. That is cheap.