How do you know when your fiance' is cheating on you?

Well my fiance' been accusing me of cheating allot lately. An I ask her to ask her mom if I can go sleep with her in our camper in the back yard. Cause she didn't have enough money to pay rent, well her family is trying so hard to break me and her up? An my fiance' been deleting all of her text messages from girls/guys. What do I do now? Any advice would be great