That one says for outside access only? Like outside the house? This one is in the laundry room.
Kinda. I did a little research on this. Many mobile homes were built with an alcove for the water heater that was accessed through an external door. This allowed for venting to the alcove from outside (a vented door), providing the necessary combustion air through the vent in the door, and, by dint of being in a separate "room", reduced the chance that carbon monoxide would back up into the house. You still need a "mobile home" water heater to meet Code when you have an outside access door, apparently, but an Outside Access one is different from a "Direct Vent" (the DV in your Warrior's model number) mobile home water heater that is rated for use in an interior closet with no outside access. It was actually all explained pretty well at the mobilehomestuffstore web site, which sells your 21I40DV for almost a grand, and an "outside access" one for about $250 less. The Sears equivalent "outside access" one is about $250 less than that, so it's too bad that Sears don't have a model that suits your needs, given their lower markup and that they only charge an extra $50 (over their regular install charge) to install it in a mobile home.
That said, there are plenty of folks posting online who claim to have put a "regular" water heater in such a situation, allegedly with no problems, but it appears that the Sears folks are only willing to work to Code, which is, of course, in a way reassuring even if it doesn't help you...
I know a guy who had a well-known professional tennis player die in his pool house when overcome by carbon monoxide from an improperly-installed gas water heater, so there's a fair argument that it's important to do this stuff right, and to have it done by a real plumber who knows what he's doing with this stuff. The bad part about gas water heaters is that their poisonous emissions don't have an odor; I am amazed how many folks don't realize that "clean burning" gas produces poisonous emissions -- even the burners on your gas stove.
One other issue that nobody has mentioned that comes up in the literature is that if you install a hot water heater that is not mobile home rated (H.U.D. safety rating) and is not the proper kind (direct vent sealed combustion vs outside access), besides being "illegal", your insurance company is entitled to cancel your coverage and perhaps refuse to pay claims.
One thing is clear from your handyman's post, and that is that he doesn't have experience putting the proper water heater in your home. I know that the person who sold you the home recommended only one guy, who sounds like he knows what he is doing, but I wonder if it wasn't a good idea suggested by the forum participant who thought you could call around to various mobile home parks and ask who they use to replace hot water heaters. If it's the same guy that you called, that's worth knowing. If they have another plumber, that might be good to know, too.
Good luck. Sorry this has sucked.