Don't get hung up on the diagonal measurement, that's the viewable screen size obviously measured diagonally. It seems that the TV manufacturers have been very focused on reducing the bezel size lately which makes the horizontal and vertical measurements smaller for the same screen size, on the LG OLED models, the bezel is almost non existent. For instance, I replaced the 48" OEM in my Monty with a 50" Samsung and it fits perfectly and 50" is a much more common size.
Apparently the VESA size (which is a standardized bolt pattern for TV mounts) changes at the 50" size; no worries, I ordered an adapter plate from Amazon for $20 that just bolts onto your existing mount. I did mount with bolts and locking nuts. One thing the RV industry does quite well is their TV mounts are very robust, I would not recommend replacing the actual wall mount.
Take your tape measure and measure the opening before shopping. Best Buy, Crutchfield, etc all have the overall dimensions on their websites. You might just find that the next larger TV will fit the opening. Who doesn't want a bigger TV?