I have the 700 watt inverter, but it was so cold. Going 50's to 60's even with fireplace. And have to run my car all night and all day long. But there are times that you need the car to buy food and get gas and this is where it gets in the low 60's.
If you haven't given up on the tank yet, get youself over to a Hardware store like Lowes or HomeDepot..... Buy some stiff Insulation "Boards".
I found some pink polystyrene sheets @ Depot ($10-20/sheet.)
I used the pink kind because it didn't crumble into pellets like the white styrofoam kind.
Then cut the panels to fit the sides&top of the tank (and bottom if you have an open stand) then duct tape the panels to the tank,
you've just made a reverse "cooler" for your tank and now you can sit back watch the temp inside the tank rise!
(I was in the exact same boat--house temps @ below 40 @ night with the fireplace going...and the 80 gallon tank was wrapped in every extra sleeping bag and blanket we had....and the temp still going down!!!)
As soon as I put the pink foam insulation on, the tank went right up to 75 degrees,
AND I was able to keep the temp up for hours at a time with just jugs of hot water floating in the tank!
I would just have to reheat the jugs every 4-5 hours--
which meant I could sleep through (most of) the night--
AND I could take the wife out for gas, food, warmth, normalcy, etc. for hours at a time during the day!
The foam saved my tank, my sanity, (and probably my marriage)!