Are the rope lights incandescent or LED? If they're giving off enough heat to raise the greenhouse temperature then it sounds like incandescent. In that case, if they're giving off an appropriate amount/type of red light then they could be affecting daylength. In other words if they're on all night then the plants could be experiencing 24 hour days. I'm not sure what that would do to spinach, which flowers when the daylength is long, so hopefully their "upright" stance doesn't indicate bolting. Tomatoes don't like 24 hour lighting, FWIW.
You mentioned you'd had a heater on at night, that was one thing I asked about earlier because it can affect growth. The night temperature should be roughly 5 to 10 degrees F below the day temperature on a cloudy day, and around 13 to 15 degrees lower on a sunny day.
