Water pressure problems

I have a 400 foot well with a new pump and pipe going to a 5000 gal water storage tank. There is a booster pump set for 60gpm that goes to a water filter that has three garden hoses attached. All three hoses have 60gpm when turned on one at a time. One hose is used for irrigation, one goes to my motor home and the third one goes to my 5th wheel trailer. The trailer has an electric water heater. The water pressure inside the trailer started dropping after two or three seconds and turning on one of the other hoses restored normal pressure long enough for a shower. That lasted for about a month then that quit working (water pressure dropped after three seconds). The water pressure from all three hoses has stayed at 60gpm. I have drained the water heater, and refilled it, released the pressure valve and pushed the red reset button. Thank you for any advice.

A typical garden hose will not flow even half of the volume you are claiming, so first I think we need to know what the true flow rate is at the manifold.