It looks like a great low budget build!
I particularly like the turquoise kitchen appliances. They look like the whole unit was taken out of a '60's or '70's vintage RV.
As far as sealing the dog house and transmission cover, butyl tape is one of the best to do the job--take them off, lay a line of tape along the edge, and then screw it back down.
As far as sealing the windshield, it depends upon where it is leaking--is it leaking because the gasket is allowing water to leak betweeen the glass and the rubber or between the rubber and the metal? If you want to do a quick and dirty job of it get some butyl rubber caulk and apply it all the way around the gasket making sure it covers both the glass and metal side of the gasket. The best way is to determine where exactly the leak is occurring. It may not even be the windshield but something higher up in the front cap.