Your water heater fails so you go to the home improvement place looking for a new one. You quickly discover that if they stock water heaters for mobile homes they cost at least $100 more than the same size and quality as those for site built homes. Even worse, they not have any and want you to special order for even more money.
Both style units look the same so Do you have to buy the mobile home rated unit?
Mobile homes may sometime be moved to a new location and may need to change from propane to natural gas or the reverse. The gas valve in a mobile home water heater can be changed to permit the use of either type of fuel. This makes the valve somewhat more expensive.
NOT THAT MUCH MORE EXPENSIVE!! you say. True.
Mobile home water heaters have also been approved by HUD for use in mobile homes. SO WHAT? you say, they both heat water. True.
However, if your home burns down or you have some other reason to file a claim with your insurance company, they will immediately check to see if you had an approved water heater. Guess what they do if the answer is NO?
That is not a place you want to go.
So swallow hard, pay the extra, and try to think of it as insurance.