Spanish often omits subject pronouns because the verb ending already tells you who is doing the action.
So Nosotros cargamos las cajas en el camión is possible, but it usually adds:
Las is the definite article, so las cajas means the boxes.
It suggests these are specific boxes that the speaker and listener can identify.
Compare:
Also, cajas is feminine plural, so the article has to match:
Yes, en often means in or on, but with movement into a place or onto a vehicle, Spanish often still uses en where English prefers into or onto.
So:
English is often more specific here than Spanish.
Because Spanish does not contract en + el.
The only common mandatory contractions with el are:
So:
Yes, and it is very natural.
There is a useful difference in focus:
Cargamos las cajas en el camión
Focuses on the boxes as the thing being moved.
Cargamos el camión con las cajas
Focuses on the truck as the thing being loaded.
This is similar to English:
Both are correct, but the structure highlights a different element.
The accent mark shows where the stress goes:
Without the accent mark, Spanish spelling rules would suggest a different stress pattern, so the written accent is needed.
A useful extra note:
In camiones, there is no written accent because the normal stress rules already give the correct pronunciation.
A simple approximation is:
More specifically:
A rough full-sentence pronunciation is:
car-GA-mos las KA-has en el ka-MYON
The given order is the most neutral one:
Spanish word order is fairly flexible, but changes usually add emphasis, topic, or contrast.
So the original sentence is the best basic version for a learner to start with.