I think this is the first of these books that I've read where the mystery really was one! Usually, the bad guys (if not everything about their motives or actions) are fairly obvious, but here there were three different possible culprits, and all had plausible opportunity to keep sabotaging the renovations at the old motel. 

I did laugh, though, at the changes the new author has made to the way the kids behave. Generally, when the kids were organising their own food with money from their grandad, it was meal after meal of bread and milk, and the Enid Blyton books I read as a kid seemed to indicate that bread and milk was acceptable food for kids, but it sounds mushy and unappetising. Especially as I'm pretty sure I remember them (Boxcar and Blyton) having bread and milk mixed together in the same bowl. Times have clearly moved on, because now when it's time to shop the kids are getting fruit and vegetables and meat and ice-cream and generally not developing scurvy or rickets or whatever it is you get from a life of soggy bread and poor nutrition. Depression, probably.