To the OP, I would try to determine ahead of time what the expectations are of the offer with more travel. Will you be expected to be on site Monday AM and stay through the end (or near) of the day on Friday. What will the work load be like once onsite.
Also, what are typical locations traveled to like. Its going to be much easier flying to a major city with lots of options than Cowtown USA where you might have only a few if just one option a day. Will the employer allow for direct flights over connections even when there is a noticeable cost difference? Will the employer pay for extras such as a lounge membership or onboard wifi? Waiting in an airline lounge for a three hour delayed flight is going to be much more comfortable than at a crowded gate with screaming babies.
If the OP is leaving late morning/early afternoon Sunday and coming home late Friday night after putting in 10-12 work days the relationship with the GF is likely to become very problematic, along with many other things in the OP's life (like trying to schedule doctors appointments, get clothes cleaning done, take care of house needs, etc.)