Hello Peter,
I am sure that someone somewhere has replaced the internal diaphragm pump with a much quieter external impeller pump. There is no reason why that cant be done. It will need some additional parts such as a separate pressure switch, but these are widley available from caravan dealers.
Diaphragm pumps are inherently more noisy that impeller pumps because that have oscillating parts which vibrate. Some caravan manufactures fix the pump to a wall or floor and that transmits the vibrations throughout the caravan. Most of these pumps come with soft rubber bushes which are supposed to reduce the noise, but some times they need more soft material to absorbe the vibrations.
If you unscrew the pumps mountings and just hold it away from any surfaces, you will see how much quieter it may be. If there is enough play in the pipes you could try holding it against other stuctrues to see if they transmit less noise.
More soft rubber or foam mountings may also help. I'm afraid its a case of trying differnt things to see what works best for you.