Stacey Lawrence's mum urges other parents to always be on guard
But Ms Lawrence said she thought it was a "one-off" and said Walker had told her he had been "vetted" in the past.
The distraught mother, who appeared with Stacey's older sister Emma Hammond, said: "There was absolutely nothing looking back that would ever have aroused any suspicion at all.
"I have analysed everything, absolutely everything, and know I don't know what his intentions were - whether he was this good guy and he genuinely did mean the things he said and did, or whether it was all a big plan to lead us into a false sense of security.
"He told me that he had been vetted at some point to marry into a family that was foster carers and things so at that point I definitely had no reason to suspect him of anything."
Stacey had been on several trips with the lorry driver and was nicknamed "baby trucker" by her family.
But at her inquest last week Northamptonshire coroner Anne Pember said sleeping in a lorry cab was "not an appropriate place for a nine-year-old girl who... should be safely tucked up in her bed at home".
But Ms Lawrence said she had seen no reason to stop her daughter from going, especially when efforts were made to make sure she had appropriate sleeping arrangements and toilet facilities.
"At the end of the day he was taking my family on, he was going to be her stepdad," she added. "It was a case of other dads take their daughters.
"I go over it every single day and think could I have seen something, could I have prevented that, apart from not letting her go that day.
"But then if I had not let her go, if he was determined to do this to Stacey he would have done it at home or any other opportunity that he had."
She said when her fiance and daughter did not answer their phones on the morning of August 29, she thought they had been "lorry jacked", until she learned the horrific truth.
Ms Lawrence said despite her hoping he had not intended what happened, it became clear her fiance had set out to commit the terrible crime, taking the strapping he used to strangle himself and Stacey from his car on the day of the trip.
She said: "The last six months I was still thinking he may have committed suicide out of guilt for what he had done to Stacey.
"I thought it might have been something that had gone wrong. I was still trying to defend him I guess, hoping that he was really really sorry for what he did, that's why he killed himself, but it would appear not. He did intend to do it."
