It's been a month since Lyle and Marie McCann of Alberta disappeared, and the police investigation so far hasn't brought family members the answers they're looking for.

The elderly couple from St. Albert, outside Edmonton, vanished July 3 after filling up their motorhome at a gas station en route to British Columbia.

Their son Bret spoke Tuesday with the CBC and issued an appeal to Travis Vader, the man police have named as a person of interest in the case.

Vader and another man were arrested July 19 in Niton Junction, about 150 kilometres west of Edmonton. Vader was interviewed about the disappearance of Lyle, 78, and Marie Ann, 77, and has remained in custody in Edmonton, charged in an unrelated matter. The other man has been released.

The police investigation continues, but McCann family members believe Vader knows something.

"I know that you’re a decent man and you have a family," Bret McCann said. "Put yourself in our shoes and do the right thing."

Days after the couple set out for B.C., police discovered the scorched shell of the McCanns' motorhome. Their Hyundai SUV, which they had been towing behind their motorhome, was later found nearby. There were few clues in either discovery as to where the McCanns might be.

Friends and family have put up posters along the Yellowhead Highway, where the couple were believed to be driving when they were last seen.

Nicole Walshe, Bret McCann’s daughter and the granddaughter of Lyle and Marie, expressed some confidence that authorities would turn something up.

"For our family there’s just no other option," she said in an interview. "We have to stay optimistic and positive and hopeful. You know, if it was me out there, I know that Grandma and Grandpa wouldn’t give up on me, and we won’t give up on finding them."

Police say they’re still getting tips and hope one of them holds the clue that will lead them to the McCanns.

Vader is charged with possession of stolen property in an unrelated case.