"Trisodium phosphate (TSP) is an inorganic compound with the formula Na3PO4. It is a white, granular or crystalline solid, highly soluble in water producing an alkaline solution. TSPs are used as cleaning agent, lubricant, food additive, stain remover and degreaser.
Trisodium phosphate was at one time extensively used in formulations for a wide variety of consumer grade soaps and detergents, but ecological problems have largely ended that practice, at least in the western world. Substitutes are not as effective." - wiki
It's around $10 for 4.5 lb. box of this stuff and that's enough for months.
You must pay attention when handling this powder. It's corrosive and has to be treated with respect. You don't need to wear a respirator or wear a face shield or anything like that but you do want to be careful where you store it and what you let it come into contact with. Do not store it in a damp location. In fact, I'd keep it sealed inside a moister proof container of some kind just to make sure it doesn't harden into a big block.
My wife adds a tiny bit to each dishwasher and laundry load. And I mean a tiny little bit - like 1/4 teaspoon. That's all it takes! Suddenly all the detergents work again the way they used to! Clean dishes and glasses! Really clean cottons out of the washer!
My question is - Do you think putting a teensy bit of phosphate back into each load would help keep the mildew out of the hoses and the door gasket?