Battery drain on 2019 Sienna XLE (wheelchair accessible)

I purchased a 2019 Toyota Sienna XLE that has been wheelchair converted on December 16. Unfortunately, I have had two issues with the battery draining and going dead in the span of 24 hours. In my area, we have been experiencing temperatures of -35 Celsius, so it made it all the more difficult to boost the car battery. After taking my car to the mechanic, I was informed that there is a draw of 1 milliamp on the battery. When I had the wheelchair conversion system disconnected, it seemed that this wasn't the source of the drain. Unfortunately, even with the system unplugged, there still seems to be a persistent draw on my battery. The mechanic is unsure of the source, so I'm hoping that someone out there can help me find a solution to this problem. Any advice or guidance would be greatly appreciated!