There is not a separate 'old piece'; this corner protector is broken off.
There are repairs that can be made. The first question is where's the old piece that goes there? Is it available to reinstall, or not? Look at the exterior portion of this forum and use the search tool.
Kidkraz was correct to point at the relevant (and current) Thread of another owner with a damaged corner (A link for the full 'Thread' is
https://www.trailmanorowners.com/forum/showthread.php?t=20293). But that new owner is in a slightly different situation, with a damaged rear corner.
Your front corner has between one and three issues. First, the entire plastic "corner cover piece cover" of the "front shell" consists of ONE piece, not two. Even if you have the broken-off lower portion in hand, I feel that you should not attach it as a separate piece: Rather, you should order a new "corner cover" from the factory Then remove the portion which remains attached (going up to the top) and then install the new one.
The second issue may or may not be present within your front corner. (Your picture is too small an under-detailed to assess.) The "street side" and "front side" vertical walls need to be firmly attached to each other BEFORE adding the "corner cover" -- the cover is
not intended to take all the stress which occurs during opening and closing. Make sure that they are firmly attached to each other, with no movement. And then:
Issue 3: In my model, the attachment and edges of the two sides (front and side) are completely covered and sealed underneath the plastic "cover" - sealed with relatively thick aluminum foil tape (overlapping from top to bottom, like shingles) - and then sealed with a high quality boat sealant.
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When you make the repair, I recommend use of 3M "boat sealant" #4200. It remains flexible over time (many years) and has good resistance to UV damage, although the white color will fade to a pale yellow over many years of exposure.
I broke my front shell "corner cover" lose several years ago, in a garage accident. Repaired in this way, it has been perfect ever since. Be absolutely sure that the drain holes remain open.