while it sure could be the mixer...
and there have been a LOT of issues with them,
be SURE the bypass valve (on the hot water line) hasn't been closed slightly.
this would give LOW pressure and LESS hot water at the shower,
but with seemingly normal function at the kitchen sink, i think.
kitchen sink bits are moen, but...
does your unit have the delta fixture in the shower?
many of the box stores carry delta parts in my hood.
if so here's the assembly/repair guide.
the mixer/handle is covered a few pages INTO the pdf file.
debris can also slow the line, but that a lot less like in a well seasoned trailer.