Jan ... if you winterize ... and don't plan on spending a lot of time winter camping, then under the trailer is a ok (but not great) place for water tanks ...
Most of today's trailer's have the fresh water tank inside the rig ... usually under a bench or a couch ... with the grey and black hanging below.
Putting the fresh water tank inside the trailer isn't for freeze protection ... it's to spread the weight
of the tank over a large area since folks usually run without fluids in black/grey but carry some fresh water with them. Less stress on the fresh water tank than if it were on hangers under the rig.
Casita's black tank in also inside the trailer, with only the grey hanging below.
Since we do a lot of winter camping ... I prefer the black tank being inside ... except for obviously the dump valve and pipe ...
The inside Black tank is less apt to freeze, especially since we're putting 98.6 F degree (37 C) fluids inside the black tank on a regular basis (which I also do several times a night).