Be careful with any of the active boosters sold on ebay and elsewhere that promise to boost your signal - you're not allowed to use them, and it's very easy for the mobile networks to track you down (or so I've read) Apparently the only one that is legally allowed to be used is the "Cel-Fi" one (around $900), which is probably not ideal for a caravan anyway.
The best option is an external antenna - and preferably a directional one like a yagi (where you point the antenna at the general direction of the cell tower). The thing is, your device will need to be able to support being connected to an external antenna, which pretty much rules out any smartphones.. Most of the decent 3g/4g dongles support external antennas (like the sierra wireless ones from Telstra), so you can use this to get Internet access, but unless you have a phone that can connect to an external antenna, you'll be stuck with using Skype for voice calls!
My parents do the grey nomad thing and I bought them a yagi antenna, which my Dad has mounted on the top of a 4m high pole, and spins it until he gets the best signal. Using this, they've been able to get a solid 2 or 3 bars where others have zero reception. These are all in remote north WA locations. If you have any particular locations in mind, let us know and I'll see if he's been there and how it's performed there.
We got great advice from Joe at
www.onewireless.com.au if you're looking for somewhere to get the gear.
Cheers