finite element method – FEM: structure is flatten when the length is long

I am trying to simulate the electric voltage distribution on a structure between lines with roughness.
I have generated mesh with the roughness I designed as below:

Above the figures are the first 100 of 100, 1000,and 10000 length unit. For me the quality of the mesh is good and is independent of length.
However, when I tried to solve the Poisson equation based on these mesh, this is what I got (here I also extract the first 100 length unit):

As shown in the figure, the Voltage is nicely solved in the 100 case, however for the 1000 and 10000 cases the roughness seems disappeared.
The code I used to generate the mesh is:

ToElementMesh@ToBoundaryMesh[
"Coordinates" -> 
 Flatten[{#[[2]], 
     Reverse@(# + {0, 10} & /@ #[[1]]), {#[[2, 1]]}} &@#, 1], 
"BoundaryElements" -> {LineElement[
   Partition[
    Range[Length[
      Flatten[{#[[2]], 
          Reverse@(# + {0, 10} & /@ #[[1]]), {#[[1, 1]]}} &@#, 
       1]]], 2, 1], 
   Flatten[{Table[2, Length@#[[2]] - 1], 3, 
     Table[1, Length@#[[1]] - 1], 3}]]}] & @ LERCoordList

where the LERCoordList is the list of coordinates of both edges.

And the code I used to perform FEM is:

`NDSolveValue[{Laplacian[V[x, y], {x, y}] == 0,DirichletCondition[V[x, y] == 10, ElementMarker == 1],DirichletCondition[V[x, y] == 0, ElementMarker == 2]},V, {x, y} \[Element] #, Method -> {"FiniteElement"}] & @ LERMesh`

Can anyone help me solving this?
Thank you.