Good day JD Cool,
A few good input on how to remedy your problem. I would like to add some additional suggestions:
Re: Sand in Tank
1. You stated that you have tried to flush the sand out. Most likely, you use the drain valve to accomplish this. I would suggest that you remove the line from the fresh water tank to the pump. Plug the line and fill the tank about one fourth full. Then remove the plug from the line and also open up the drain. By adding the additional line - i.e. pump line, it will "aid" in removing the sand. It my take several tries to get at much sand out of the tank. This will also remove any sand deposits that lay around the tank's connection to the pump.
2. As suggested, either use the Camco external blue filter or install a whole house filter in the coach. This will prevent further containment like sand to get into your fresh water tank.
Re: Existing Filter
3. Looks like this is not doing a good job. I would just eliminate it. You could consider using an external ICE Maker filter. This is small and available at most Big Box Stores (i.e Big Orange, Hardware, etc.). Easy connections. You will have to adapt the 1/4 inch connections on the filter to a larger size to accommodate the larger size hoses used for the tank to the pump. Some of these filter use screw fittings for connections, other use push on. I recommend getting a filter with screw fittings so that you can purchase a brass "reducer" to accomplish this.
Best or Luck
2008 Newmar Kountry Star 3960, Spartan K2 Chassis, Cummins ISL 400HP, 2014 Chevy Captiva