The opinions on this topic each time it comes up fall in 1 or 2 general answers.
1. We leave the slide open all the time. Never had a problem, they are made to do this.
2. We close the slide every time we leave camp. It takes 30 seconds. By leaving the slide open, your risk is greatly increased for damage and accelerated deterioration to the slide system.
I do no. 2 when the campout is over. Why?
If you have ever lived through slide floor rot, deteriorated seals or rusted solid slide racks you will know why.
Slide Floor Water Damage (Long with lots of pics) We bought our camper 2.5 years old. We mostly weekend camp, not on a permanent site. The TT sat on the dealers lot almost 1 year of that time with the slide open for display.
Water off the slide roof runs down the side of the slide, wicks under the slide, then breaks the capillary action free and falls to the ground. Any hole or exposed protrusion to the waterproof membrane or the ends of a coated slide floor system allows water to get in. Once in it cannot get out. A year or more later the rot just keeps growing.
If you have a slide topper this helps divert some of the water off the slide, however they too become totally sun baked brittle being open all the time.
The slide seals are affected by the sun. Over time they break down after so much exposure. If you are only using the camper 20% of the season, then 80% of the season the seals are degrading with the slide left open and you are not even using the camper.
If your camper has a rack and pinion slide drive system, rust is the enemy of those systems. Many owners manuals say to open and close the slide every month etc. This is often needed so the rust patterns gets disturbed and the system not freeze up from corrosion.
Everyone's situation is a little different. If you want the longest life from your slide system, close it when you not using the camper. While they are built in most cases to give long life and good service, they are not as resilient as the rest of the main camper.
If you are parked in an area of trees, pine cones, falling twigs etc, and no slide topper, it is best to sweep off or at least look at the slide roof before retracting all the debris into the slide seals.
Hope this helps and good luck.
John
2005 Ford F350 Super Duty, 4x4; 6.8L V10 with 4.10 RA, 21,000 GCWR, 11,000 GVWR, upgraded 2 1/2" Towbeast Receiver. Hitched with a 1,700# Reese HP WD, HP Dual Cam to a 2004 Sunline Solaris T310R travel trailer.