Sewer

Yes, valves and tanks are enclosed. Pretty much screwed til weather is above freezing.

Really nothing for salt to harm, the valves are stainless, but I would try a $10 heat gun first. Mine from Harbor Freight really throws out the heat. Good luck.

Salt is a corosive and hard on your sever valves. As previously suggested a har dryer mcht do it. Then figure out why it froze. Need more slope?

Brooks I've read about using about 3 gallons of RV antifreeze, one guy said try to get it warm before you dump it in. Recall saying dump 1/2 gallon down the kitchen, bathroom and shower, follow by 1/2 gallon of boiling water down each of the drain followed by another 1/2 gallon of antifreeze. Other say use a couple boxes of table salt and hot water.

Are your tanks enclosed at all or totally exposed? Are you running the propane heat? Might keep the fan on and the cabinets open to air circulates.

No, but a well-placed hair dryer might 'unthaw' it.

Anti freeze is not freeze unthaw good luck