Yes, replace the LP detector. There will be a DATE on the back side when you remove it. The smell is wet/rotted wood or carpet. The fact you smelled it after a hard rain means that there has been water penetration at the slide out over the years. Most Monaco slide seals need replaced every 5 to 7 years. They just wear out and fail to give a complete seal. If you pull the Cabinet drawers and access the area in the sidewall slide hole you will probably see the wet or rotted area. Then use fans to dry it out. IF NOT in a slideout, you need to check the window seals. Doug