Water barrels are the best for collecting heat. Other thing is to simply fill you house with as much stuff as you can. I also cover my greenhouse with a frost cloth. Helps keep heat in. You can also do hoop house inside your hoop house.

I use water barrels, and the more the merrier. I also close the door a couple of hours before sunset, so I can trap the warm air inside before it cools too much. You can throw frost cloth over the hoop house like mksmth just mentioned, or do a smaller hoop house inside your hoop house....like a low tunnel inside a high tunnell, and you can use plastic or row cover for the lower tunnel or the hoop house inside the hoop house. I know someone who used a hot glue gun to glue together 2-liter soda bottles in a round cylinder shape they could place around 1 tomato plant. It works similar to a wall-o-water but obviously is much longer. You probably could surround your tomatoes with a 'wall' of milk jugs filled with water or you could use 20 oz. bottles of water, letting them absorb heat all day and release it to warm the plants at night.

I have left flats of tomato plant in my greenhouse when the temperature has dropped as low as 24 or 25 degrees outside and they were fine. Generally, though, if we are going to go too deeply into the 20s, I just carry the flats of tomato plants into the house at night. Luckily for me, we rarely get that cold at night once the tomato plants are large enough to have been moved from the light shelf indoors to the unheated greenhouse. My shade cloth is Aluminex and one reason I chose that specific type of shade cloth is because they said the metallic material would help keep the greenhouse slightly warmer at night. I believe that it does because the greenhouse seems warmer at night now than it did that first spring when I didn't have shade cloth at all.

About the only thing I do when a cold night is expected is to throw a medium-weight row cover over the table of tomato plants before I close the doors in late afternoon. I've never had a plant show any damage from being in the greenhouse on a cold night.

I have used a heat lamp or a work light I have that puts off a lot of heat. I also have painted empty milk jugs black and filled them with water. My hoop houses are only 4' x 8' and maybe 4' tall at the center, so milk jugs may not work with the size of your hoop house.

Here are a couple of videos of a guy who used Christmas lights to heat his small hoop houses.