When you say it is running out the side, do you mean out the fresh water tank fill inlet on the side of the unit? If so, then the problem is likely the 'check valve'. Normally this is an inline valve near where the city water line and the 12V pump line meet. It is a one way valve to prevent city water from back feeding into the water tank. Yours is probably stuck open due to age and sediment. You might locate the valve, it will look similar to an inline fuel filter ( it won't have a handle) and maybe tapping on it will solve the problem. Another temporary solution would be to just cap that line if you are only going to use city water. Third option is to not use city water, but just run off the 12 v pump.
If the water problem is someplace else, you will need to find the source of the leak. Are you using a water pressure regulator from the city water tap? If not, then higher than normal pressure will find weak points in the system, especially on an older unit.
Larry Day, Texas Baptist Men volunteer
'13 Silverado LT 3500HD D/A CCSB 2wd, custom RKI bed
'11 Crossroads Cruiser 32MK loaded