It wont hurt the bike running it up to 65 in 4th or even higher, matter of fact My old 83 Venture could run up to 75 in second gear these motors are high RPM's monsters my 83 model had no rev.limt.
But by understanding your post you said it lugs in 5th gear doing 65? to me thats not right, It should be smooth as silk at 45-65 untill you want to pass then go down a gear. Without seeing the bike or hearing it run its hard to say? but for me its does not sound right.