Try googling the manufacturer and model of you current hot water heaters to see what those temp settings mean. Yes, 115-120 degrees is best--not only in terms of efficiency, but any hotter and someone could easily be scalded by too-hot water coming out of the faucet or shower.
Here are some ideas for reducing your hot water use:
1. Shorter showers and/or shut off water when soaping up.
2. Dishwasher--run only when full of dirty dishes. Use dishwasher's most economical setting. Rinse (or soak) dishes/utensils before washing in dishwasher.
3. Clothes washer--again, only run when washer is full--partial loads waste hot water. Wash warm or cold and rinse cold.
4. Turn the thermostat down when it's cold. Last winter I had the thermostat on 60 while I was there, mid-fifties when I was out. (I have a drafty house which I am currently spending $$ on fixing!) I turned the thermostat up to 65 if I had company over (aren't I a good hostess!) Do what you can to plug up drafts, add insulation, caulk windows--even that shrinkwrap they sell to put over your windows in the winter helps.
5. Proper hot water heater maintenance: drain to get rid of sediment at least once a year (if you don't know how to do this, I'd highly recommend looking up instructions on the internet, or better yet, have someone who knows how to do this show you). This will greatly extend the life of your heaters and improve efficiency.
Hope this helps