70% shade covering, especially in the winter, will not let enough light through to produce tomatoes. Also, a single layer of any polyethylene will not offer enough heat retention on cold/freezing nights, especially for tomatoes.

Thank you, I'll have to replace that.

I have a 55 gallon drum Pianted black in there, filled with water. I dont know if that will help with the heat.

Would a double layer of the polyethylene hold enough heat or is a cheap hoop house not going to be realistic for keeping enough heat without adding some form of a heater?

Double poly, with an air space in-between layers, will reduce heat loss by about half, but on cold nights, you would still need a heater for tomatoes. The same is true for any type of structure, including a hoop house.
You did not mention the size of your greenhouse, but a 55 gal water drum will have released most of the stored heat by midnight.

The hoop house is 16x13. I'm not sure how I'd go about adding an air space between the two layers...I'm going to have to do some more research.

Not in any kind of rush, I have a sun room I've been growing the plants in so far, but looking to get something out back where they can get full day sun and more room.

6-mil poly has an r-value of .6 and an inflated double layer has an r-value of 1.4. Both of those numbers are very low. For a double layer, you need a small blower that takes outside air and blows it between the two pieces. But if you already have electricity, a small electric heater would probably be advisable.