Checking MOT history

I'm looking for a reasonably priced motorhome and like to check the mot history of potential vehicles on the DVLA web site. Sometimes I see something like "underside completely undersealed, unable to check". Does that mean that it could be a rustbucket underneath? You would think the tester wouldn't be able to complete the test if they couldn't check the condition. Isn't the whole point to prove the vehicle is safe?