Wholley crap.....temperature swings..to cold or hot. Mobile/modular homes are not a shed out in the back...
Built same as a house, except for basement. Been in mine for 6yrs now and a new one awaits in Kamloops.
Anyways on to his question. I had a 90 for awhile. They have similar floor joists as a house, plus two large steel beams running length wise. I would still check with the builder or as someone mentioned have something professionally installed for a tank as large as a 180.
But then most do that under their floor for basement houses also. One good thing, again as mentioned, you can get under them and easily build a cribbing support to the ground.
Some like mine, have the 3/8th gyprock flex walls and are fully supported by cribs under the beams. Others, such as our new one, have regular 1/2in gyprock and require cement pillars under the beams.
So guess it depends on which type your purchasing, used or new?
Ours heats the exact same as a house. And air conditioning keeps it cool in the summer, same as a house and all at a cheaper price, depending on size. They come now in every size imaginable. I have seen 2500 sq. ft. homes to 800 or 900sq ft. Ours is 1216 sq ft.
The new place is a bit smaller at 1060 sq ft though. But my tank is only a 35g cube type