The pump reverberates through the entire plumbing in the camper, making it sound much louder than it really is. Actually, the pump itself is very quiet. But the pulsing of the water beating against the pipes is what you're hearing.
In the past, someone posted a modification they did to their water pump. It was quite impressive. They used several feet of flexible hose rolled up in a coil (for space) and then attached the flexible hose to the normal RV water system. The principle was to use that flexible hose to absorb the sound of the pulsating and not in the actual pipes of the camper, that touch walls and floors, and every spot it touches works as an amplifier, amplifying the sound.
This was by far the best cure ever posted on these forums for noisy water pumps.
As long as your pipes are connected to the pump, and those pipes are touching anything in the structure of your camper, that sound will be amplified. That is the problem.