I have a 2020 Xplore 304 and while it has the underslung heater I can's say for sure if the heater and controls are the same as yours. We don't run the heater at night so we'll not have encountered your problem but if your controls are the same as ours there are some fault codes that can be displayed. I'll attached some info at the end of this post.
I probably wont have an answer for you but perhaps some background info may help so....
1. Are you using the Electric or Gas to run the heater?
2. If Electric which setting are you using 500/1000/2000
3. What fan speed have you chosen? Is it on Night setting or the Frost Setting or a speed above these?
Here's the manual info on the heater, which you may have anyway.
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