There are for sure several ways of estimating that number, but a quick top down approach can be resumed in 3 steps
1. Estimate the total population of the US: ~300M (I guess it's already a bit higher by now, but this number makes calculations easier and works out as a proxy)
2. Divide by the average size of an household (which should be somewhere between 3 and 4, let's assume 4): ~75M
3. Assume a percentage of households that like/do camping at all: 50% >> ~~37.5M (I don't have any particular insight in the US, so this number might be a bit off; if you're from the US just relate to your day-to-day surrounding and experience)
4. Assume a percentage for ratio of people that actually hold a caravan: 20% >> ~7.5M (same disclaimer as before)
What really matter is you're able to come up with the process/way to get a ballpark figure, not that the numbers are exactly the right ones.