Although they're big, they're still young. They may think that Go Potty 'to your place' ... really means "when we are there with you to watch.... and admire...
I don't have a definitive suggestion, but how 'bout some brainstorms...
1. Walk out with them, say "Go potty" and when they get started, walk out... within sight.
2. When they begine to go elsewhere, say "Go Potty" to interrupt them, and guide them to their spot.
3. Make sure the area is cleaned, but with something to smell left by. I notice that my dog needs something to read... He sniffs around, finds something interesting he left last time, and he positions so that nose is over the "reading material" and tail is over the new "target area." ... So, yes, there is a 3/4 dog length between target areas, and "lightning" never strikes the exact same spot twice.
4. Although he may pee anywhere, he usually doesn't poop where he has peed.