I realize it has been sometime since the last post about this subject, but I ran into this while I was looking for a solution for the DHC3 Otter for P3D5.3. Somebody may have already mentioned this somewhere, but I could not find it. Anyways, I couldn't get the Otter to work for more than a couple of minutes after launching it in a ready to fly state. There were a number of things that did not work.
So here is what fixed it for me and now it works great. This may work for other Milviz aircraft as well, but I do not know since this is the only one I have.
Solution, I entered the following lines into the Dll xml file located at C:\Programdata\Lockheed Martin\Prepar3d v5
<Launch.Addon>
<Name>XMLTools64</Name>
<Disabled>False</Disabled>
<ManualLoad>False</ManualLoad>
<Path>XMLTools64.dll</Path>
<DllStartName>module_init</DllStartName>
<DllStopName>module_deinit</DllStopName>
</Launch.Addon>
I could not get the weather radar to work, but I do not use it anyways...so no loss there for me. If you have trouble installing the otter or other aircraft originally designed for v4 into v5, the instructions found on this video may be your solution, I followed them long ago and now anything that was originally designed for v4 will install into my v5 location by default.
https://www.youtube.com/watch?v=gTsDNwhAsVE&t=91s