كيس النوم بالخيمة.

Because Levantine Arabic usually leaves out the present-tense verb to be in simple statements.

So:

• كيس النوم بالخيمة
  literally looks like
• the sleeping bag in the tent

but it means:

• The sleeping bag is in the tent

If you wanted past or future, then a verb would normally appear.

It is an iḍāfa construction, which is a very common Arabic noun structure.

• كيس = bag
• النوم = sleep / sleeping

So كيس النوم is literally bag of sleep, which is how Arabic expresses sleeping bag.

This is similar to how Arabic often builds compound nouns with noun + noun instead of using an adjective like English does.

Because in an iḍāfa construction, the first noun normally does not take الـ.

So Arabic says:

• كيس النوم

not:

• الكيس النوم

The definiteness of the whole phrase is controlled by the second noun, not the first one.

Because the second noun in the iḍāfa is what makes the whole phrase definite.

So:

• كيس النوم = the sleeping bag

If the second noun were indefinite instead, the whole phrase would be indefinite too. In everyday usage, you may also hear كيس نوم for a sleeping bag or just sleeping bag in a more general sense.

Because the preposition بـ attaches directly to the noun.

So:

• بـ = in / at / with, depending on context
• الخيمة = the tent

Together:

• بـ + الخيمة → بالخيمة

This is completely normal in Arabic spelling.

Here, بـ gives a location meaning, so بالخيمة means in the tent or at the tent, depending on context.

In Levantine, بـ is very common for location in everyday speech.

You could also say:

• في الخيمة

and that would also be natural.
Very roughly:

• بالخيمة = very colloquial, everyday Levantine
• في الخيمة = also natural, sometimes a bit more explicitly inside the tent

In many situations, both work.

A natural pronunciation would be something like:

• kīs in-nōm bil-khēme

A few notes:

• كيس → kīs
• النوم → in-nōm / en-nōm
• بالخيمة → bil-khēme

The ن in النوم is a sun letter, so the l sound of الـ disappears in pronunciation and the n is doubled.

In Levantine, it can absolutely be a full sentence.

Arabic often makes present-tense sentences without is/are. So this structure is enough:

• كيس النوم = the sleeping bag
• بالخيمة = in the tent

Together, that gives a complete statement.

Because this is a very natural Arabic word order: first mention the thing you are talking about, then say where it is.

So:

• كيس النوم = the topic
• بالخيمة = the location

That is a normal and common way to build sentences like this in Levantine.