when my light ,fan, and heater are on my gauge runs under 11 volts i thought it was an alternator but when i replaced it there was no change and i have good grouds help,please.

 

Well first when you say 11 volts is that using a volt meter or using the gauge in the car. They are not very accurate for a starters.
If it really is 11 volts and you have checked your earths then check your alternator, it should be charging at around 14 volts but it could be a low capacity unit or faulty. Either way you are then running your car and accessories on the battery alone and yes it will drop to 11 volts with all that switched on