Voltage across terminals with motor on and off?
If the alt is charging that's probly not it.
Motor off - should see 12-12.6 or 12.7 depending on charge state
Motor on - should see 13.7 to 14.4 as charge rate from alternator.
On the winnies the slides are operated by the chassis batteries, step too. Sensor for step is house batts though, they're all a little different.
You need to isolate the parasitic drain on the chassis batt circuit. A shop can put a ammeter in line with the batt to see if there is indeed drain, but that's kinda obvious so just a matter of finding the offending device or wiring. A true short would probably blow fuses, so it seems more like something turned on that's pulling power.
Dash radio has the switch for house or chassis source? Switch it over to house?
That's quite a bit of juice to kill an optima overnight.