I'm confused. Is it your mom who has the problem with the tanks or your fiance???

If it's your mom, well, they're your snakes. Keep them how you feel is best. If she tells you to get rid of them because you moved them into tubs, send them to your fiance's until you two get a place together. When's the wedding? If its close, keep them in the tanks til you move out with your new spouse. The tub and uth will keep til then. And you can make do in the meantime with the tanks. Many many snakes are housed successfully in tanks (mine included) and there are tricks for dealing with the humidity. It's not worth damaging the relationship with your mom over this.

If its your fiance who's giving your grief over this, perhaps you should rethink things.....