Furnace question

We just returned today from a 9 day trip to the upper penninsula in Michigan where the temp was in the 30's on 2 nights and we stopped using the small electrical heater we had and turned on the furnace. The furnace worked great but i noticed something weird. While the heater blower was on, if i turned on any light in the camper you could here the blower go lower like an overload to the circuit. I did have the fridge and hot water tank on. Is this normal? (no fuses were blown) Thanks

[Bill]

It is normal for the voltage to drop (and hence for the fan speed to drop slightly) when the load on the circuit increases. It turns out that the human ear is very sensitive to pitch, so you hear a small change (speed drop) that you wouldn't notice on any other circuit - lights, for example.

Bill

Thanks, I was a bit worried that a small light could cause a voltage drop.