Adrian,
There's a few things working against your configuration:
1. If you van has aluminium wall panels, then these will adsorb most of the TV signal - so you are much better off with an external aerial
2. If you have a Status "mushroom" or "pod" aerial mounted on the van roof, then these have a reputation for only working in strong signal areas
3. An analogue laptop card does not have the same characteristics as the TV tuner fitted to a quality TV - the sensitivity is down a bit, signal to noise is worse, but with a good signal they do work.
4. Fully support Sonic's advice - a quality external aerial is so much better than an internal aerial
5. Remember that camp grounds are usually tucked away in a picturesque valley, or way out in the countryside where signal strength is quite low.
A bigger aerial is always better than a small aerial and a booster, a bigger aerial and a booster is better yet
Robert