Learn Dutch: Wij slapen in de grote slaapkamer.

In Dutch, adjectives usually get an -e added to the end when they come before a noun. Since slaapkamer is a de word, the adjective takes the -e ending. Notice the spelling change too: groot loses an 'o' when it becomes grote to keep the same long vowel sound.

Could I say we slapen instead of wij slapen?

Yes, absolutely! Wij is the emphasized form of "we". You would use wij if you want to stress that we (and not the guests or the kids, for example) are sleeping in the big bedroom. If you are just stating a neutral fact, we is more common.

Is slaapkamer a combination of two words?

Yes, it is a compound word made from slaap (the stem of the verb slapen, to sleep) and kamer (room). In Dutch, the gender of a compound word always comes from its final part. Since it is de kamer, the combined word is de slaapkamer.

If I wanted to add the modal particle even to say "We are just sleeping in the big bedroom for a bit," where would it go?

Modal particles like even usually go in the middle of the sentence, right after the main verb. Here, you would say: Wij slapen even in de grote slaapkamer. This makes the action sound casual or temporary.