The pluming lines are in the wall where the head end of the shower sits. Just like the other bath. If the head is on an exterior wall, and you have a one piece shower or tub you will need to open the wall from outside the home. If you have a shower or tub with a separate wall surround you can open the wall on the inside of the tub and replace the plumbing and surround.
Opening the wall from the exterior side can be a challenge with lap siding. I have re-plumbed about 30 homes. When we find this situation, with the shower or tub on the exterior wall and it is not exhibiting any plumbing problems
, we change out the plumbing under the floor and leave the plumbing in the wall as is. In many cases, considering the time skills and funds to remove and re-install the siding - if it ain't broken don't fix it.
Read these free articles for more details:
Replacing a Manufactured Home Bathtub
Understanding and Installing Manufactured Housing Waterlines