hi...
it being in the garage section makes it a bit easier to repair, since it is usually open and not much to remove. but you will need to remove the flooring, as far back as you can, isolate the rotten wood and cut the wood out. then install new wood.
the best way to do this is to replace the full width of the trailer, cutting out so the join of the new wood falls on a cross bar under the floor. ideally then, its good to overlay the entire floor with 1/4" luan. this will require removing all the flooring though and installing new, as usually the old is damaged when removed.
the walls.... they will be harder. depending on if you have the metal sided or fiberglass sided trailer. if it is a metal sided trailer, the interior panels will pop off, they are stapled on. you can then replace them as needed. you may also need to replace some interior framing if it is rotten.
if it is a fiberglass sided trailer however, the interior panels are laminated and glued to the styrofoam core. removal is slow and tedious as you have to carefully pry them free and in bits, taking care not to damage the styrofoam behind them. then reglue new panels. advantage is, it will most likely be aluminum framing so no rot to the frame. it is possible to just remove as much damaged panel as possible ,then fill the now exposed area with paneling piece followed by a complete overlay of the entire wall, attaching new panels over the old to hide the repaired area.

it is not a job for the faint hearted and will require a fair amount of time. the floor has to be repaired properly and as structurally sound as poissible since it needs to take the load of your toys running over it. cut out, patch and overlay is the best and strongest method to repair the floor. utilizing as much of the existing cross supports as possible.

before you do anything though, you need to locate the source of the leak and fix that. it is a good idea to do this before repairing, as you can then use a water hose and starting from the top of the trailer, sprau it with water and see if you can see where it comes in and then fix that area. never assume the first leak you find is the only leak however, continue to test until your sure.
if water doesnt show up te leak, it is recommended you take it in to a dealer and have them perform a seal tek test on it, which is a maching that pressurizes the interior, exposing leaks to the outside when sprayed with a soap solution.. test is usually not that expensive and will show up leaks not always visible with water.....

Thankyou for your question.. please remember this is the answer page and to click accept if you are satisfied with my answer so i get paid for my efforts.. Please also note, accepting my answer does not terminate our correspondance, you may continue to reply after accepting and i will work with you until the problem is solved.
thankyou and please let me know if you require further assistance.....
:Nick: