Before doing anything, check on all the screws on your water pump, especially the 4 long screws holding on the pump propeller assembly. I had a similar problem with a previous trailer and discovered that 2 of the screws (bolts) were completely missing, the 3rd one was barely hanging on and the 4th one was far from snug tight. I found 1 of the missing bolts laying on the floor and the 4th one I never found. I think it fell out and dropped through the floor where the hoses came up through the floor.
I ended up buying a new pump, identical to the original, except a newer version and never had a problem with back flow again. I still have that old pump, but it's still missing a screw. It works fine, and maybe someday I'll figure out something I can use it for that's not installed into something permanet. I'll never trust it to not leak now.
Anyway, check on the bolts and screws on yours before doing anything. It's possible one or more is just loose letting water back flow past the seals. That's the FIRST thing to check. If all is well there, then consider the check valves.
To plant a garden is to believe in tomorrow.
2019 Montana High Country 375FL
2014 Chevy Silverado Duramax, 6.6L Dually