Our Nordyne/Intertherm furnace and A/C (that I rewired way back when) had a 10 or 15 VA transformer for heat and an add-on box with a standard 40VA for cool, the thermostat was 2 wire, standard R and W, closed for heating, open for cooling, with a large DPDT rocker switch for heat/cool on the add-on box that poked through a hole cut in the furnace's front cover.
I re-wired it for a standard 4 wire T-stat because the smaller Xfmr buzzed so loud it could be heard at both far ends of the trailer at night. As per my other thread from years ago I retained all the safeties and even added a 3 amp fuse for the Xfmr secondary.
I still have the old setup in my mobile home I use for camp. 2 wire with 2 or 3 relays in the box. Still have the old GE baby bottle fin coil and all for the A/C.