I agree with Alex. Generally speaking altars are fixed and immobile, but I might handwave the limitation if it's the right community and the right deity. For example Hei Feng* might have an altar on an abandoned wreck floating inside a permanent hurricane. An unusually devout traveling Varisian caravan might have an altar dedicated to Desna (the god of travel) in one of their family wagons.
That said, dragging an altar of Erastil along on your next dungeon expedition is unlikely to net you anything other than a mildly displeased deity.
*Disclaimer: Only very mildly familiar with Hei Feng so this might be an awful example, only using this as an example since his subdomains include Catastrophe, Ocean, and Storm.
Strictly speaking no, I think kudaku's suggestion of a Desnan Altar in a Varisian Caravan would probably be one of the most appropriate exceptions to house rule.