I had the same problem with my 35' Georgie Boy Pursuit. It was especially bothersome traveling in the cold weather, with cold air blowing on my feet and legs.
The big trick is to find where to spray the foam as you can't see the leaks. You will need a buddy to help. I tried doing it at night using a bright light under the dash and looking in the engine compartment, and doing it the reverse...shining a bright light from the engine compartment and looking under the dash. Neither worked that well but did show a couple of places.
The best way was to use my air compressor and a long wand tool blower. Just have your buddy move the wand around the fire wall and you listen for leaks under the dash. You can use a length of tube to your ear to pin point the source if needed. You don't need to be really specific, just close enough to know the area. When you determine a location, just foam that area in the engine compartment. There will likely be more than one.
Mine isn't perfect yet, but it is a great improvement.