Soil test is the way to go. I recall you have sandy soil. So your going to add small amounts in april and may. General rule of thumb is a coffee cup of fertilizer per inch of trunk size. Sandy soil I'd do half a cup twice.
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Plain old 10-10-10 will do ok.
Lack of growth can have alot to do with lack of moisture. IF you can align your trips to camp when the trees need it. Or give them water at the end of your weekend up there can make a big difference. A little miracle grow won't hurt either. Little hole in a bucket slow watering method works great.
Besides a soil test being worth its weight in gold, mulching around the base of the tree.
Extending a good growth spurt before a tree goes into water stress probably does more than fertilizer for most folks.