Furnace blowing cold air only

IF you measured .47V across the switch wires at the switch. The switch is bad, replace it.

Did you check to compare the circuit board in water heater with the one in furnace for possibly switching them?

well fully charged batteries, lots of gas just no ignite
totally lost
Unless theres a way to force ignition it
looks like a trip to the repair shop

showed .45v coming out

With fan running ....
Check the voltage OUT of Sail Switch to circuit board (white wire to yellow)
White is +
Yellow is grd

IF you have DC Voltage then circuit board could be the issue

Incoming voltage is 12.75
fan is freely spinning

so both batteries charged to 12.75
still will not ignite
fan just runs and runs

Check furnace incoming voltage.
Check the fan to make sure it is spinning freely. Do this by spinning fan with your fingers. Of course with the fan shut off electrically!
Why is only one battery being charged?

ok so now 1 battery charged to 12.85v would not ignite it.
There is gas to the stove.
I did have the protection cover off to look at the sail yesterday but after putting back on last night it did fire up hence the rush to think it was solved.

2 batteries, Group 24, both brand new this year