I had to replace the slide subfloor in our previous travel trailer due to water damage. I decided to cut the floor and only replace about 1/3 of it since it only had water damage in the corner. I used some metal mending strips to hold the two separate pieces together so you wouldn't feel one sag as you walked across them. If I had to do it again, I found there are metal brackets for plywood that are meant to help mend two pieces together, that may have been easier.
Overall, if you have the tools and patience, it can be a DIY job. There are several videos on YouTube where people have shown how they did it.