I have a mountain cabin. In the winter, we go up and use it in freezing
weather.

I am going to redo the plumbing so that we can haul water up in the winter
and use it through piping pressurized by 12v. RV pumps.

When we leave, I will have a low point drainage valve.

However, there are P trap shaped hoses (the toilet and sink supplies,
namely) where water will accumulate, and not drain by merely opening a drain
valve.

We do the usual thing of putting RV antifreeze in the toilet and p traps.

I am assuming that there is a Schrader type valve I can plumb into this so
that I can used compressed air to blow out the remaining water to avoid
bursting. The interior of the cabin gets down to the teens during the
winter when no one's there. The outside gets about ten below.

What do I ask/look for at the hardware store?

Thanks in advance.

Steve