I'm not a fan of adding stuff like Seafoam to my fuel or oil. Using a top tier gasoline and changing the oil on time with a quality oil do a much better job of keeping the engine and fuel systems clean and running correctly. Besides, once the gunk builds up and you loosen it the crud can clog small openings.
That being said, I would definitely use Sta-Bil in the fuel tank of a vehicle that sits for any length of time. Even with Sta-Bil it only lasts for up to a year.
I would think about what you want to do with the MH. If the plans are to start using it again, bring it to a mechanic for a good tune up and have it thoroughly checked out. If all you do is start it up every year or so, I'd consider selling it while it still runs and has some resale value. You could consider selling it as is or getting the tune up and trying to get more money for it. Not sure which way is best, that's something you'll have to figure out.
Best of luck!