How would a guy insulate the floor when using post and pier foundation, and how would you insulate water supply and waste lines?
You insulate the floor of a post and pier foundation the same way you would a crawlspace foundation.- between the joists.
For water lines, etc again do just what you would do for a perimeter crawlspace - bury pipes in insulation where possible, use split foam insulation sleeves where exposed and then get a strategy for cold weather when you won't be there. This is determined by climate — leave a faucet dripping, build an insulated box around the pipes and leave a light bulb on, wrap with heat tape where most exposed.