fresh water back flow

First I do have an outlet to fill the freshwater outside the trailer. The water is overflowing and dripping out this outlet unless I open the drain.If the back flow valve is in the pump I will probably replace the pump. (the trailer is over 10 years old). Any suggestions on pumps? Will use the pump infrequently. I will be putting this trailer on my seasonal site this fall for good replacing my 25 year old Holiday Rambler(gotta love the HRs). I will use the pump when the park shuts off water for the winter for a couple of weeks.

Is your rig setup with a fill valve to put water into the fresh water tank?

Since the pump is not pumping water AND is feeding back into the FW tank it may be the diaphragm itself is damaged. If you're handy it may be possible to repair the pump but it's not a HUGE expense to replace the pump with a new one.
Good luck / Skip

Assuming it is a Shurflo pump, the check valve is inside the pump. The check valve is comprised of a number of rubber flapper valves. Sometimes, sand or grit causes these check valves to leak. Disassemble the pump and inspect for grit before you give up on the pump.