Day to day, you'll be doing your own cooking and washing up, much as you would in a self-catering cottage, so it helps to bring a few kitchen basics like tea towels and washing-up liquid rather than assuming everything will be there. Bedding is usually provided, but it's worth checking your specific booking, since this varies park to park.
Day trips are where staycation budgets often quietly slip, so it's worth planning a mix rather than assuming every day needs an admission ticket. A coastal walk, a free stretch of beach, https://www.visitbreckland.org.uk/places/united-kingdom/norfolk/ashill/camping-touring/brick-kiln-farm/ or a wander round a market town costs nothing beyond fuel, and mixing these in with the odd paid attraction keeps the week varied without every day carrying a ticket price.
On the budget side, self-catering works in your favour with a bigger group. Cooking most meals in the caravan rather than eating out as a party of eight or ten keeps costs sensible, and splitting the shopping and cooking between the adults spreads both the effort and the spend. A shared static caravan holiday, or two units booked close together, usually works out considerably more cost-effective than the equivalent number of hotel rooms plus regular restaurant bills.
The first thing to check is whether you can move outside a Friday-to-Friday or Saturday-to-Saturday week. Many parks release short breaks, three or four nights rather than a full week, and these tend to have more late availability simply because fewer people search for them. If your dates can flex by even a day or two, you'll usually see more choice.
If the park has a clubhouse, pool or entertainment programme, these are usually optional rather than something you're expected to take part in every evening. Plenty of guests use a park purely as a comfortable, well-equipped base and spend most of their time out and about along the coast or countryside nearby, coming back only to sleep, cook and relax.
Off-peak months help enormously too. School holidays and the height of summer are when demand is tightest, so a late booking in May, September or October usually has far more choice than the same search in late July. The weather isn't guaranteed either way, but a coastal or countryside break in early autumn can be genuinely pleasant, with quieter paths and shorter queues wherever you stop for lunch.