Yeah, with a $30k budget, you are really looking at truck campers if 4x4 is a must.
This is something where a wheel mounted electric assist motor for the front wheels would be a cool option (yeah unsprung weight). Obviously, not directly applicable but I've seen them for golf carts to make them 4x4. You just need a little extra traction for a couple minutes on occasion not true off road capability.
Another thought: Do most of the Class C's come with a limited slip diff? If not, adding a manual locker and selecting tires suited to traction in sand might get you 70-80% of what a full 4x4 would give you (most of the weight is on the rear axle anyway). Add a beefy winch with a sand anchor and I'm betting it would do good on most level sand beaches.