Turn the water off at the water spigot. Turn your water heater off. Secure all you camping gear (out of site, out of mind), remove any food that may spoil, take dirty laundry home when you leave the camper for a several day.Empty your trash containers.
There is still an open debate about retracting slide-outs for seasonal campers when owners are gone. Some leave them out, some pull them in. I would be one to pull them in when leaving for a few days and also retract the awning. There is just less of a vunerability with the slides in, and the awning can easily get blown away in a high wind. So always pull your awning in if you're not there.
You want to clean your camper (inside) after every use, rotate clean sheets, pillows and blankets as needed.
Why all the cleaning when you leave? Because those odors will get trapped in the camper and when you come back the next time .... wahoo!!!! So you want to eliminate everything dirty and smelly when you close up shop.
Same is true for your holding tanks. Dump them and clean them out (if you have full hook-up you'll have no problem adding water to your black tank to keep it flushed out good.) Regardless of those who say their black tanks don't stink... um ... yes they do.
When you push that peddle on the toilet and that valve opens! Wahoo Yippee!! The aroma of a skunk will permeate and you'll be breaking out the Febreeze! So, keep them emptied and cleaned out when not in use. Flush good, always.
Don't keep anything of value in your camper unless you are right there, even on a season site. So if you plan on keeping a computer, shot guns or skeet guns, or cash-money in it ... well .... don't! It takes absolutely NOTHING for someone to bust open the door and take anything that's not nailed down. So, simply do not leave anything in the camper you value, unless you are there with it.
This is about all I can think of. Everything above is true for seasonal campsites as well as for those who travel.