You may want to try this all out at home before going "field live". While I have NOT put together a system as you describe, others in this forum have.
I recall from those other threads in which the Sawyer filter is used that the filter MUST be backwashed or filled with water using a home faucet before it will work at the lower pressures inherent in a gravity feed system. And, the filter must be kept filled with water (and not allowed to freeze) if it is to used again before having access to the higher pressure available at a home faucet.
This is not meant to be knock on the Sawyer filter, but it means that you will be carrying the weight of a "full" in-line filter with its water inside.
Perhaps others who have first-hand experience with the Sawyer in-line filter can confirm or clarify my impression.