Sewer Line Freezing

Well the first thing you should have done is insulate the pipe....

https://www.grainger.com/category/pipe-insulation/insulation/plumbing/ecatalog/N-r6b

including any part above ground, but in unheated space, conducting your heat trace heat back up the pipe, away from where you need it (where some water must remain).

I don't know SH#* about sewers (sorry, I couldn't resist) but for a first level answer you need one of the following:
1) more heat
2) better insulation
3) global warming

Well, I'm not real hot on the third one so let's go with the first two. Anyone trying to help will probably want to know details about the heat tape you installed. They will want to know if the area is something like an active parking lot (insulation not reasonable) or a pad out back that is not used in the winter (4x8 sheets of styrofoam seem good when the choice is no toilet). I don't know what else is important but anything you can think of related to heating, cooling and insulation is probably good to add to the post.

Is the pipe PVC or cast iron? What state/country are you in?

You should have used a John Deere. Nothing runs like a Deere!

Bury the line deeper, as has already been suggested.

Bury the line deeper and don't remove snow from above it...if it snows there. Here in Maine we put things like this down at least 4 ft below ground level.

the trick is get below the frost line with any uninsulated pipe.

You have the option of boxing up around the pipe (after insulation), to decrease cooling rate due to extreme temperatures outdoors.

Heat coupled from the housing unit floor should help maintain the box temperature above freezing.

• Put skirting around the trailer, and otherwise create a sealed envelope between the bottom of the trailer and the ground below. Consider addind a layer of insulation inside the skirt. This will also make it easier to heat the trailer.

• Don't turn off the heat to the trailer, just set the thermostat to a low temp to save energy when not occupied.

• When the forcast is for a hard freeze, leave some water running. Alternately, during a hard freeze, set up a slow trickle of denatured alcohol or brine...perhaps poke a small hole in the bottom of 5 gallon bucket and put it where is drains to the line.

• Avoid any low sodium diet.

• Add additional antifreeze heating cables/tape to the line.

There are many examples of fine American out houses that sustained pioneers. They operated well, without the need for sewage piping.

Skirt the trailer as stated in #8 below.

Apply 12" minimum of sawmill grade sawdust, which will provide several years of heat from decomposition.

You may have to water the sawdust every few months.

Also apply treatment for termites if you have them there.