We really need to know the exact Casita you have. Besides 2003 SD we need know the length. I'm going to "assume" it's a 17 foot
one for the following as it's the most popular one.
Casita changed the back window size in 2003 so your back window could be either one I'm guessing.
But first what is wrong with your unit?
Your 2003 came with the Dometic 2453 that is a manual one that has a big advantage over the current ones, that is it does not require 12 volts to operate.
Might not sound like much but the new ones require 12 volts DC even to operate on propane
or 120 volts AC so low battery
means it won't work period and the manual ones are not being made.
If you have the small back window, like our 1999 the refrigeration part must be removed from the box before it will fit (not a simple task) through the door. Do you want a new refrigerator
taken apart just to get it in? I just replaced the refrigeration unit on our refrigerator
and it all had to be done inside the Casita, not fun but possible and my refrigerator
So now what's wrong with yours? It's much easier to fix these then it is to replace them.
Plus Casita did not do such a good job on the install, check out this "how to make the refrigerator work better" thread. Might just need some venting modifications.