My 3 yr old won’t sleep

My 3 1/2 year old boy will not sleep unless we’re sleeping next to him. I can lay next to him for an hour or more and he will sleep but the moment I get up he will also and he’ll cry. We live full time in an RV due to my husbands job and they have their own beds, we’ve tried sleeping him in his bed, in the sofa and put his mattress in the “living room” floor which is inches from out bed. We have him, an almost 2 year old and a 1 month old so there is no way we can fit all 3 kids with us which is why we want to sleep the two oldest on their own. My daughter does great most of the night she’ll wake up 1-2 times but will go right back to sleep when we tell her. We’ve asked him if he’s scared if he wants a night light or complete dark if he wants a toy or bear with him a specific blanket or pillow. Nothing has worked! We’re beyond exhausted from trying to sleep him and waking up with the baby my husband and I get a good 2-4 hours a sleep every night! Wake up early in the am for his job and I stay with the kids but my oldest won’t nap during the day so I can’t sleep either. I just want us all to get some more rest because I can imagine it’s difficult for him too. I’m not sure if I should just let him cry it out until he gets used to it? Also this started waaaaay before the baby came along 😢