The problem is that if you don't stir up the sand and dirt in the bottom of the fresh tank, it will just stay in there. Running it through the plumbing with the pump won't take it out that well, and risks clogging faucets and shower heads.
Need a large diameter drain valve and a lot of water flow to get it out. That's why filling the fresh tank through an external sediment filter is a good idea. Keep it out before it gets to the tank.
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'The Silver HamShack' (2007 International 22FB CCD 75th Aniversary model)
Multiple Yaesu Radios inside and many antennae sprouting from roof, ProPride hitch
2012 shortbed crewcab 4x4 Toyota Taco TV with more antennae on it