Concerning the chassis, it is really going to depend on owner maintenance. Take oil changes as an example. The six year old model would only be on it's second change using miles but sixth using time. Reverse for two year old coach. Maintenance records would be a big up check for either.
House usage is the other side of the coin. 30K miles in two years is a lot of use but the 10K miles coach could have been parked in one spot and used all summer. 30K miles in two years is 82 miles a day. Could the coach have been a rental or such?
Two years is in a bit of the golden time for a used coach. First owner takes the care of the post sale punch list of factory installed problems plus the depreciation hit. A six year old could have a couple of owner installed upgrades but there could be a few maintenance items coming due very soon such as roof seals. You're going to have to dive deep into each with a detailed inspection before making a choice.