If he’s parking over common property he’s parking on common property – it doesn’t matter where his wheels are. The next time the strata manager or rental agent writes to him it should be to explain that he is breaching a by-law which means he is also breaching the terms of his lease.
It’s a shame you can’t work this out amicably but this person seems to have no consideration for his neighbours so it may take a warning that he could be evicted if he doesn’t pull his head (and his truck) in.