Unfortunately, I believe that threshold is attached to the door frame. I tried to remove mine to tighten up the strut attachment and ran into the same issue - would not come out after the screws were removed. I did not want to break the door frame loosed so I engineered another way to tighten up that strut post. You may not have that option.
Leak on a new unit? Any idea where the water is coming in? With the powered step right under it should provide some protection?
On mine, the outside is a silver insulation board with tape on the joints. When I got the unit last year I looked around the entire area to make sure nothing looked loose or caulk broken, etc. Steps and step wells generally get beat up from the wheels throwing debris so I include this in my twice yearly inspection.
Thanks for the comment to check underneath. I just had the step mount fixed as one side was attached to light sheet metal. Maybe ok for a 100lb person but not me at 230 lbs.
So will check that out today underneath!
Also can see recent seal repair near door, above hinge. Looks like water could have run down side and in at this point.
Wood screws were severely corroded and maybe water issue was early in RV's life. Transport from up North on roads with salt. Have seen salt corrosion in other places. And know RV was moved from Indiana to Texas in December 2018.
Learned now to check entire RV after each trip to see if any new leak points!
good luck once you get all in place use the best materials and caulk everything. I use flex seal and once it sets all is good
Montana 2020 Thor Vegas 25.6
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