The typical older mobile home has one bathroom and adding another to create an en-suite will add significant value to your home. Since you already have a bathtub, you will likely want to make the en-suite a 3/4 bath with a shower stall, and because mobile homes are short on storage space, planning for generous storage will make your home more functional. Designing the bathroom back-to-back with a walk-in closet will enable you to remove the closet doors in your bedroom and provide extra space for either a comfortable chair or additional bedroom furniture - chests of drawers or an armoire/entertainment centre that will create a second living space so that you can watch Coronation Street while your husband is channel surfing between boxing matches and ultimate fighting.
Keeping the addition close to the existing structure will simplify the creation of an attractive roof line. This will be further simplified if the addition is built a step down from the existing floor. To make the best use of bedroom space, the door to the addition should be near a corner. The addition need be only about six feet deep and fourteen feet long, divided in the centre. Passing through the doorway into the addition brings you into the closet, with storage space along one entire side and half the other side, with the door to the bathroom against the outside wall. You walk past your clothes to get to the bathroom. For economy, rods and shelves can be left open, or bi-fold doors can be added to conceal the storage completely. As you enter the bathroom, the toilet will be in front of you, with the shower stall and vanity against the wall common to the addition and the existing structure. This leaves space along the outside wall for windows in both the closet and the bathroom, both of which should have frosted glass, at least in the bottom half of the window. This configuration puts most of your plumbing on an inside wall. The wall with the toilet stack will have to be well insulated. Consider in-floor heating beneath a ceramic tile floor for extra comfort and luxury on a budget.