شاحنة السحب في موقع الحادث.

In Arabic, the present-tense verb to be is usually not stated.

So:

• شاحنة السحب = the tow truck
• في موقع الحادث = at the accident site

Together, they naturally mean The tow truck is at the accident site.

This is called a nominal sentence in Arabic.
If you wanted the past tense, you would normally add كان or كانت, for example:

• كانت شاحنة السحب في موقع الحادث = The tow truck was at the accident site

Because شاحنة السحب is an iḍāfa construction, often called a genitive construction.

In an iḍāfa:

• the first noun usually does not take الـ
• the second noun determines whether the whole phrase is definite

So:

• شاحنة السحب literally = truck of the towing
• naturally = the tow truck

Even though شاحنة has no الـ, the whole phrase is definite because the second part, السحب, is definite.

This is very common in Arabic:

• باب البيت = the door of the house
• موقع الحادث = the site of the accident

No. السحب here is a noun, not an adjective.

It comes from the verb سحب, which means to pull or to drag.
The noun السحب means something like pulling, dragging, or towing.

So شاحنة السحب is literally something like:

• truck of towing
• or towing truck

English usually prefers tow truck, but Arabic often builds this kind of idea with two nouns together rather than a noun plus adjective.

It is another iḍāfa construction.

• موقع = site or location
• الحادث = the accident

So:

• موقع الحادث = the site of the accident
• naturally: the accident site or the scene of the accident

Again, the first noun does not take الـ, and the second noun makes the whole phrase definite.

A careful pronunciation is roughly:

shāḥinat as-saḥb fī mawqiʿ al-ḥādith

A few helpful notes:

• شاحنة = shāḥina(h)
  The ح is a deeper, breathier h sound than English h.
• السحب is pronounced as-saḥb, not al-saḥb, because س is a sun letter, so the l sound of الـ assimilates.
• موقع = mawqiʿ
  The final ع is the Arabic letter ʿayn, which has no exact English equivalent.
• الحادث = al-ḥādith
  Here the l of الـ is pronounced, because ح is not a sun letter.

It would be:

شاحنةُ السحبِ في موقعِ الحادثِ

Here is why the endings are like that:

• شاحنةُ: nominative, because it is the subject of the nominal sentence
• السحبِ: genitive, because it is the second part of an iḍāfa
• موقعِ: genitive, because it comes after the preposition في
• الحادثِ: genitive, because it is the second part of another iḍāfa

Also notice:

• شاحنةُ does not take tanwīn
• that is because a noun at the start of an iḍāfa does not take tanwīn

In normal everyday reading or speech, these short case endings are often not pronounced clearly, especially at pauses.

Yes. شاحنة is grammatically feminine.

You can tell from the ـة ending, which is the tāʾ marbūṭa.

This matters for agreement. If you describe it with an adjective or refer to it with a pronoun, that agreement is feminine:

• شاحنة السحب كبيرة = The tow truck is big
• هي في موقع الحادث = It is at the accident site

Many vehicle words in Arabic are grammatically feminine, even though they are not biologically female.

It can be understood as a complete sentence in Arabic:

• شاحنة السحب = topic / subject
• في موقع الحادث = predicate

So it means The tow truck is at the accident site.

That said, in real life, the same wording could also appear as a caption, headline, or short report label, where Arabic often uses compact phrasing. So it works well in both ways.

في is the standard preposition meaning in or at, depending on context.

So:

• في موقع الحادث = at the accident site

Arabic often uses في where English uses either in or at. The exact English translation depends on what sounds natural in context.

Yes. شاحنة السحب is understandable and good MSA, but Arabic can vary by region and context.

You may also see or hear things like:

• سيارة سحب
• شاحنة قطر
• other local terms in different countries

So if you encounter another expression for tow truck, it does not necessarily mean this sentence is wrong. It may just reflect regional or stylistic variation.