Hi,
we have a 95l Dometic RM2350. A 9kg gas bottle lasts us between 22 and 28 days. We use the gas for the fridge and a small amount of cooking.
If you haven't used the van for a while, the gas may have been purged from the gas lines. In that case, they can be a pain to get started. The usual cure is to turn the gas on at the bottle, go inside, light one of the gas burners on the stove to ensure you have gas in the lines.
After that it can take a while to get the fridge lit.
Note, that this is my experience with our fridge. Other fridges will behave differently.
Yes, they are safe. The gas section of the fridge is sealed from living area of the van. Correct installation will ensure there is a hole in the floor at the rear of the fridge to allow any gas leaking to drain out through the hole since gas is heavier than air. The fridge has a flame our detector, so should the flame go out, the detector turns the gas off.
We have one gas bottle in the van and carry a second one in the car. After a bit of practivce, they can be changed over in about five minutes.
take care
Mike