Some of the high-end locksmiths should have your trailer key blanks in stock. You can just have a duplicate made. I went to all of the local hardware and dept stores around here looking for a key blank and didn't have luck but did find one at LOCKSMITH place and was able to get a duplicate made.
You might check with a local RV dealer and see who makes their keys... Most of RV TRAILER keys are all the same - especially the storage doors etc...
I too keep all of my spare keys in my truck which has a pushbutton key entry from the driver side door.
We have found the RV Trailers are not a very secure thing... We do not keep any valuables in ours.
Roy Ken