You need to check the coverage of each provider in the area where you want to use it. Some service providers are better than others, or you may be in a spot where there is no coverage at all.
but as others have said, make sure there is coverage where your caravan is sited. We have t-mobile dongles for work and where some of us live (and work from home), the connection keeps dropping out which is really irritating.
I've tried out two dongles and neither were any good in my area , despite PC world saying that the signal strength was OK . Make sure that if they don't work you can take them back without any hazzle.
Are you staying at a caravan site, if so check to see if they have wireless, if they do and your Laptop/Pc is wireless enabled you should be able to connect to the internet without having to spend money on a dongle............