As you have no Wifi then the EE box is no. They are 3G anyway
For a legal signal booster for your Van you are looking at a starting price around £300.
As I say, check what signal other campers have on what network. Going to be a lot cheaper than buying a booster which may or may not work. Remember it has to pick up a signal at your location. You can't boost what is not there.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.
You make a very good point about WiFi in caravans enlli!!
For the benefit of the OP, then.. basically the signal you see on your mobile is the signal you will get via Virgin Mobile (or indeed any other EE MVNO - PlusNet, AsdaMobile, take your pick).
EE may have additional coverage on 800Mhz that (if present) will certainly improve fringe service, otherwise you probably are looking at choosing another network. You're perhaps not aware that there are only 4 physical networks in the UK - EE, 3, Vodafone & O2. Every other provider simply piggybacks onto one of these. And for very valid technical reasons that enlli has hinted at, 3 & EE use additional 4G bands that their resellers don't have access to. VF & O2 don't have those issues, so for example Voxi Mobile will basically be the same coverage as Vodafone or Giffgaff cf O2.
Asking around other campers is definitely the way to go.