Rear Storage Box
The first thing to keep in mind, is to not exceed the weight rating of the M/H. There are 2 possible reasons you don't have much storage, 1) poor design or 2) the unit is too close to its maximium weight already and CAN'T carry much stuff. Remember to include you, passengers, fuels, food, water and some black/gray water weights when figuring out how much you will be transporting.
Once you find that you have enough weight capability, you need to make sure you don't overstress the structure (hanging heavy weights on materials which won't support them) or change the driving characteristics (changing the center of gravity forward/rearward or upward). Options to consider inside would be under furnishings (like the beds, couch, etc) and adding forms of organization (hooks, shelves, drawers, etc) to existing storage spaces. Also, if the unit is high enough,you may be able to make space for light items on top of cabinets or the refridgerator, or just install some light cabinets/shelves in out of the way wall locations, if any.
Also, things like paper plate, bowl, napkin and tissues dispensors can be mounted to the underside of shelves/cabinets.
Outside, your choices are on the roof (cargo pods), off the front (usually limited to a bike or two or the like) or rear (bikes, folding chairs, etc at a minimum). If you have a hitch in back, you can easily add a hitch carrier (enclosed or open) up to the weight rating of the hitch. If no hitch, you may be able to mount some light storage to the bumper, or add a hitch if the frame extends all the way back.
Your last option would be to look underneath the unit and see if there is any place to add compartments. Be aware of heat (exhaust system), blocking maintenance, ground and operational clearance, and the like.
If you are towing a vehicle, store stuff in or on it. If you are not towing a vehicle and can tow something, consider a small trailer.
In all cases, add the weight of the storage solution to the dry weight of the M/H when figuring out how much stuff you will be carrying, and don't go over the vehicle's maximum rating.