How do you firm up old camper cushions?

If they are made of foam inside, they cannot be firmed up. Foam has a limited life expectancy, and it loses its elasticity after several years. Decay happens faster if they are exposed to extreme heat. You may need to get new inserts for them. Call a camper store that sells campers and RVs and ask them. Start with the manufacturer of your camper if you can reach them.