Caveat: I am not a security pro. I just read a lot of security articles on Ars Technica.
So, here's your bucket of salt.
I often use public wifi networks for all sorts of things... but with some defenses in place.
My bet would be that on campground wifi you're more likely to run into someone snooping around in the data flowing through the network (it's almost as easy as picking up a phone in the other room to listen in on the conversation) than you are to encounter someone actively trying to hack into your device (quite a bit more work, like a spy sneaking into a building to plant a bug). Really, there's just easier ways to get access to a device than trying to break in from the network (mostly by getting you to install something).
The most likely problem with campground wifi is likely to be web site logins. Two examples:
Many non-profit sites, like this forum, use an unencrypted connection (but most business sites, like your bank, use encrypted connections). It means that on this site, your browser is sending your user name and password to the forum server as plain text, which someone snooping on the network would be able to see. They might then try those credentials on more important sites, like Facebook or Gmail or a bank. If you re-use logins, you can easily set yourself up for identity theft.
If you connect to a network run by a malicious operator - for example, if it LOOKS like a campground or hotel wifi but is actually run by somebody else - they can inject code into web pages. Hotels do a harmless version of this, sometimes, where they show a banner or login page, or inject their own ads. A bad actor could use the same technique to lay fake user name and password fields over the real ones on a page, so that when you try to log in, you've sent your credentials to a hacker.
Easy things to help avoid these problems: 1. Never use the same password for more than one site. 2. Verify the name of the network with the campground staff. 3. Use a browser, like Chrome, which warns you if the connection is not secure (I don't know how Safari handles it). 4. Use a service like Encrypt.me (or other VPN) to automatically encrypt all web traffic to and from your phone when you are not on a trusted network.
The other things you're doing over wifi are probably safer than logging into web sites:
If you are getting updates to your operating system or virus software, you're probably fine. There are ways to hack that, but they're pretty sophisticated and you're unlikely to run into them unless you've been targeted by the NSA or something.
If you're doing searches on Google or watching videos on YouTube, someone might be able to see what you're doing. So, treat public networks like you're surfing in public and keep the embarrassing stuff for trusted networks.
Email is never really secure. Even if you have an encrypted connection to your email server, once it leaves your server to head off to the recipient's server, it's mostly transmitted as clear text. Just don't put private stuff in emails - assume someone could read them if they wanted to. Pick up the phone and use your voice if you need to keep a password or account number safe.
For backing up files, you want a service that encrypts the files before they ever leave your device. The good news for iPad/iPhone users is that iCloud backups are encrypted before they're transmitted to the cloud. So, I probably would go ahead and let iCloud backups run over a public network. There are similar applications available for PC (like Spider Oak).