Instead of replacing the pump which is expensive, or replacing the check valve in the pump which is a pain in the a__, or even a ball valve which requires you to take some action (opening it up) when you want to use the tank water, why not spend $10.00 for a 1-way check valve that will fit inline in the pex tubing.
They are available at Home Depot, Lowes or any other home improvement store, or any plumbing supply house, not to mention Amazon, eBay and other online sellers.
Fast, Easy and Cheap Permanent solution.
2013 Wildcat Sterling 32RL
2008 Chevy Silverado 2500HD LT Diesel
2001 Cougar 281 EFS (sold)
1973 Layton 23' (sold)