Welcome to the forum. If this was a "new to you" fifth wheel, the dealership should have inspected and serviced it before pickup. If they did their job properly, then all of the systems "should have been checked and should be operating properly".... That said, your husband didn't damage the water pump system by overfilling the fresh water tank. If, when you turn on the pump, you can hear it running and there is water "gushing out of the city water fill port", then the bypass valve in that fitting is malfunctioning. Again, "IF IT'S A NEW TO YOU TRAILER" then it should be covered under a buyer's warranty for a few days, contact the dealership and ask.
As for the Keystone Owner's Manual, you can download a copy here:
https://www.keystonerv.com/owners/owners-manuals/
The above manual is a very basic manual that covers all of the Keystone brands and all of each brand's models, so it's a "bland, generic booklet" with very little "model specific information"....
As for each of the appliances and components in your trailer, you can find the model number and brand name for each, use GOOGLE to search for that specific owner's manual. Just google the model number and the term "owner's manual pdf".
Any specific questions you have, post them on the forum in the appropriate section and hopefully a member will respond with the information you need.
Good Luck and welcome to Keystone and to RVing.