Appreciate all the suggestions, I am no plumbing expert but the plumbing shop I use,owner is. They do not carry ABS and the nearest box store is 80 miles one way, joys of rural living. There is a hardware store with selective common parts.
1. I explained what I was doing at the local pluming shop, they set me up with parts. Being zero pressure I am not worried about glue failure. Yes it all runs down hill and after pulling dump valve the pipe drains. We have lived on septic tanks 40 years and not much beside water goes down the drain in gray tanks. With what ever residual water remains in the 1.5" drain pipe I don't believe freezing to be an issue, the slide valve is in the enclosed belly above the modification.
2. money was not and issue
3. The 1.5 PVC runs down hill into the 3"
4. If it fails I'll post it, but I'm confident it will still be in place when I decide to trade.
I am not an expert but an avid DIY who lives rural, reads and goes to YouTube when I don't know how. Lived at one time in a log home from standing dead on the property, off grid 8 miles from the county road. (that was a learning experience) I am almost finished with the 8th and hopefully last, full house remodel on a 1600 sq ft 3Bdr 2 bath. that had been a rental and goes on the market when done.