Might be easier to drop your tanks in place before you put in the floor. Speaking from experience, it's "fun" to lay on the ground and shove the fresh water tank into place with your feet......
Propane lines definitely need to go on the OUTSIDE of the belly pan. We put Pex over the lines to protect them from road debris hits when we installed them.
We ran our battery lines in the belly pan because we moved the location of the converter and battery, which are not near each other. It was easier to go underneath the floor than try to snake things through the trailer. Otherwise all our electrical is in the walls.
Water lines should be inside the trailer to protect from freezing. (Just reiterating what has been said.)