There are a few threads on this already. You should be able to fill this cavity. See the response from Jefferson on his costs to replace the entire floor of a 16x80 home.
On most items you will get more competitive bids from mobile home specific repairmen. Approach your local dealer and ask who performs their warranty work or find someone through Craigslist or Google to test them out in small jobs and up the ante with increasing trust in their abilities.