A couple of solutions you can do...
Just replace the cut RV cable with a new 30A 25-30 foot cord with molded RV 30A plug in the end... You will have to locate the 120VAC 120VAC Distribution Panel and wire this into the main 30A Breaker. It would not be a good idea to just splice the new molded cable and plug into the existing wiring that was cut off. having the stubby connector is great if you are ever going to run a cable to the generator setup or connecting a long 10-gauge 120V Extension cord to the garage 20AMP receptacle using a RV30A-15A adapter...
Another you can do if you have enough of 30A cable coming out of the door access door you can also connect an enclosed RV 30A SOCKET too.
You could easily then purchase a RV 30A MALE/FEMALE 25-foot extension Cord and just plug it into the short stubby connection. The beauty of this is you can store the extension cord in a safe place when not in use.
A better thing would be to install a RV30A RV Connector Socket in the access door hole and wir this to the 30A Circuit Breaker. Then you can purchase an RV30AMP cord that mates this socket and use it as well. Again the beauty is you can store this cable in safe location but is more difficult to inst...
Would look something like this
The kit looks like this...
Only three wires to hookup - can't be all that hard to do haha...
Roy Ken