Most times, to determine if a very slow change in state of voltage is occurring, it is sufficient to leave the
Oscilloscope "free running" in "auto trigger" setting, with any convenient time/div setting.
With the 'scope input DC coupled, this will present a horizontal line across the screen.
As the voltage state changes, the line will move up or down the escreen.
If you really have to see the actual waveform shape, set time/div so that the sweep takes several times the anticipated time of each cycle, & still in "free -running" mode, wind up the intensity control until you can see the spot tracing out the waveform.
Note that this is not a "locked" display, & it may drift across the screen over time.
Another good idea is to reduce the lighting level
You may be able to record this on a digital camera in either the "B" setting, if it has one, or as video.
(we used to use a Polaroid camera)
Analog storage 'scopes were not all that common in the past, but people got by with the above work-arounds.