I don’t remember much about it (and I have a hunch I may have asked before, but until we get a reasonable search feature I don’t think I can find it) but I do remember this much
[ul][li]The space ship was a Space Shuttle that was supposed to be futuristic, but I guess they didn’t have a budget[/li][li]They land on Earth in The Future[/li]IIRC, civilisation has collapsed or devolved[/ul]
Planet of the Apes, the television series?
No, this was later and there weren’t apes in it. The idea was that this ship of explorers returned to Earth after some centuries and found cities overgrown and technology forgotten (except when the Bad Guys had some dangerous artifact, of course).
Sounds like maybe Strange New World?
That looks very familiar. I remember seeing John Saxon in something. But I want to say that the one I’m thinking of was shown after 1976, and I’m pretty certain the ship was an unmodified Space Shuttle model.
Was it Ark II? I used to love this 
Oooh and it’s for sale on Amazon!
Product Description
The 1970s children’s show ARK II reflected that decade’s ecological concerns with the sci-fi adventures of three human scientists–leader Jonah (Terry Lester) medic Ruth (Jean Marie Hon) and tech genius Samuel (Jose Flores)–who along with their talking chimpanzee Adam travelled in their high-tech vehicle to right the wrongs of the environmentally ravaged 25th century. This collection presents the fun conscientious and charmingly dated series in its entirety for a total of 15 episodes.Format: DVD MOVIE Genre: TELEVISION/SERIES & SEQUELS Rating: NR UPC: 787364725090
Not Ark II.
Genesis II sounds familiar, but it’s from 1973.
Well, the 1977 series Buck Rogers started with Buck in a Shuttle-looking spaceship, and finding the future Earth to be a post-WWIII environment with most of the old civilization destroyed.
And in between Genesis II and Strange New World was Planet Earth. All connected with Gene Roddenberry. You had:
Genesis II – 1973; starring Alex Cord. TV movie; pilot.
Then Planet Earth – 1974; starring John Saxon. Series based on Genesis II, with some changes.
Then Strange New World – 1975; again starring John Saxon. Third attempt to get the series off the ground.
Maybe I’m conflating the Shuttle from Buck Rogers with the plot of Strange New World.
Logan’s Run had overgrown cities. (But no Space Shuttle.)
Except for the “landing on Earth” part, it might be Starlost.