[Mikendebbie]

We have a 2018 3921FB which is similar to your unit. I don't remember ever bumping the trim overhead - but for what it's worth I went out and measured the clearance in our unit (33") - and we replaced the OEM mattress with a foam mattress and a topper. The top of our bedspread is even with the top of the bedside shelf, which in the screen shot below from the Montana website you can see that the OEM mattress is below the shelf top.

I will warn you to watch out for the cabinet door over the in-command controls (left side of the screen shot). Many folks here have made the mistake of accidentally leaving it open during set-up or take-down and hit their head on the door and it hurts. I hit my head on it two or three times, but when DW hit her head I have been batting 1000 for closing the door ever since!