I reently installed a Water Softener in my Horizon 40AD. I did not want to be lifting the heavy water softener in and out of my compartments. I positioned the Water Softener in the first compartment forward of the Water Bay and ran two water hoses out the side of the compartment. I used a horizontal electrical cover to make a door for the hose access. I used quick disconnect water connections for the hook-up outside the coach. I am now using this set-up and it connects quite easily and I never have to lift the water softener.
See photos below:
Water Softener installed in compartment.
Water lines running to coach and water source.
Close-up of access door and water hose quick disconnects.