RE: One way valve on hot water in shower
I’ve been lurking on this thread waiting for someone to mention theShowermi$er, but no one has. It does exactly what the OP wants IIRC.
RE: Black Tanks Won’t Flush
According to the Google you only have one black and one gray tank.
Specs for 2018 Heartland - Bighorn
Floorplan: BH 3870 FB
(Fifth Wheel)
Holding Tanks
Number Of Fresh Water Holding Tanks1
Total Fresh Water Tank Capacity 65.0 gal.
Number Of Gray Water Holding Tanks1
Total Gray Water Tank Capacity 90.0 gal.
Number Of Black Water Holding Tanks1
Total Black Water Tank Capacity 45.0 gal.