In the parts of Africa that BP's come from the average humidity level year round is about 80%. There is no health risks of humidity levels that high as long as the tub isn't damp (damp tubs can cause belly rot and spur mold growth)
71 is low, 75 is the minimum, and 78-82 is ideal. There are a few things you can do.
1. You could rig your heat lamp so it heats up the tub (keep it far enough away that it doesn't melt the tub.
2. Get an oil filled heater and use it to raise the temperature of the room right around the tub
3. Keep your room/house a few degrees warmer.
You are feeding the right size, and frequency. Young BP's should be offered food every 4-7 days. If your BP isn't eating double check your temperatures, and do not handle at all.