A lot of good info here. 150baker's info is good stuff, but I would only recommend installing a mobile home approved gas water heater as it is code and when selling your home, a home inspector could throw that up. Also, if there is ever an insurance related WH fire issue, the insurance company may not cover the damage if it is not an approved water heater. Not sure on that, but that is what I have heard. That code does not apply to electric water heaters, only gas WHs and furnaces.
Enforcement could be a regional thing. Inspectors here used to approve non MH gas water heaters, but not any more. But if you pull a permit, I would think they would say it has to be MH approved. JMO
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All information and advice given is for entertainment and informational purposes only. The person doing the work is solely responsible to insure that their work complies with their local building code and OSHA safety regulations.