المرتبة الهوائية داخل الخيمة اليوم.

In Arabic, the verb to be is normally not written in the present tense.

So:

• المرتبة الهوائية داخل الخيمة اليوم means
• The air mattress is inside the tent today

This is a very common Arabic pattern called a nominal sentence.

You only usually see an explicit was / will be / is not when the tense or polarity requires it, for example:

• كانت المرتبة الهوائية داخل الخيمة أمس = The air mattress was inside the tent yesterday
• ستكون المرتبة الهوائية داخل الخيمة غدًا = The air mattress will be inside the tent tomorrow

So the absence of is is completely normal.

Because الهوائية is an adjective describing المرتبة, and in Arabic adjectives agree with the nouns they describe.

That means the adjective matches the noun in:

• definiteness
• gender
• number
• usually case as well

Here:

• المرتبة = the mattress
• الهوائية = the air / inflatable one

Since the noun is definite with الـ, the adjective must also be definite with الـ:

• مرتبة هوائية = an air mattress
• المرتبة الهوائية = the air mattress

Because مرتبة is a feminine noun, and Arabic adjectives must agree with the noun they describe.

So:

• مرتبة is feminine
• therefore the adjective must also be feminine: هوائية

Compare:

• سرير هوائي = an inflatable bed/air bed
• مرتبة هوائية = an air mattress

The adjective changes depending on the noun it describes.

It means inside the tent, and it functions here as the predicate of the sentence.

More specifically:

• داخل = inside
• الخيمة = the tent

Together they form a location expression.

A useful way to think about it is:

• داخل الخيمة = inside the tent
• literally something like the interior of the tent

In this sentence, it tells you where the air mattress is.

It is often taught like a preposition because that is how it works in translation, but grammatically it is a bit more noun-like than a simple preposition such as في.

You can think of:

• في الخيمة = in the tent
• داخل الخيمة = inside the tent

داخل often behaves like a spatial noun meaning something like interior/inside, and the noun after it is in the genitive:

• داخلَ الخيمةِ

For most learners, the practical takeaway is simple:
داخل + noun means inside + noun.

Because داخل is followed by a noun in a genitive relationship.

So in full case marking, you would have:

• داخلَ الخيمةِ

Here:

• الخيمةِ takes the genitive ending because it comes after داخل in this construction.

This is similar to how many Arabic location expressions work.

اليوم means today, and it adds a time meaning to the sentence.

So the sentence gives both:

• where the air mattress is: داخل الخيمة
• when this is true: اليوم

In other words:

• The air mattress is inside the tent today

It is an adverbial expression of time.

Because Arabic often places time expressions flexibly, and putting اليوم at the end sounds natural here.

This order is very common:

• subject
• location
• time

So:

• المرتبة الهوائية داخل الخيمة اليوم

But Arabic word order is flexible, and other orders can also work depending on emphasis, for example:

• اليوم المرتبة الهوائية داخل الخيمة
  = Today, the air mattress is inside the tent.

The original sentence is a neutral, natural order.

Not naturally from this exact word order.

The sentence begins with المرتبة الهوائية, so that is the topic/subject-like element:

• The air mattress ... inside the tent today

A natural English translation is:

• The air mattress is inside the tent today

If Arabic wanted to strongly foreground the location, it could start with داخل الخيمة instead.

In fully vocalized formal Arabic, it would typically be:

• المرتبةُ الهوائيةُ داخلَ الخيمةِ اليومَ

A simple breakdown:

• المرتبةُ = subject (mubtadaʾ) → nominative
• الهوائيةُ = adjective matching the subject → nominative
• داخلَ الخيمةِ = location phrase functioning as predicate
• اليومَ = adverb of time → accusative

In everyday writing, these endings are usually omitted.

A careful pronunciation is approximately:

• al-martabah al-hawāʾiyyah

A few helpful notes:

• ة at the end of المرتبة is pronounced like -ah in pause
• الهوائية contains a hamza: hawāʾiyyah
• when reading naturally in context with case endings, the endings may be heard as well

If you pause after the first phrase, you will often hear:

• al-martaba al-hawāʾiyya

No. مرتبة can have more than one meaning depending on context.

Common meanings include:

• mattress
• rank / level / status

In this sentence, because of الهوائية (air / inflatable) and inside the tent, the meaning air mattress is clearly intended.

So context is what tells you which meaning is correct.

Yes, but the meaning is slightly less specific.

• في الخيمة = in the tent
• داخل الخيمة = inside the tent

Both can work, but داخل emphasizes the interior more clearly.

So:

• المرتبة الهوائية في الخيمة اليوم = The air mattress is in the tent today
• المرتبة الهوائية داخل الخيمة اليوم = The air mattress is inside the tent today

The second sounds a bit more specific about location.

Yes. It is a complete Arabic sentence.

It has everything it needs:

• المرتبة الهوائية = what we are talking about
• داخل الخيمة = where it is
• اليوم = when this is true

Arabic often expresses ideas very compactly, especially in present-tense nominal sentences. So even though it is short, it is perfectly complete.