1.) look around your kitchen and see what you use every day...coffee pot/blender/toaster/whatever as appliances...plates/glasses/utensils...we have sets of this stuff to keep in the trailer all the time
2.) What do you like to eat? I've made a 'load and/or buy' list that we use and 'check off' before each trip...everything from coffee to oatmeal to meat to bread to water to soda to that other frosty beverage I like...don't forget meat (if you bbq), charcoal or a full propane tank for the grill, spices and sauces for the meat, lighter fluid, long grill lighter, etc.
3.) What will you need for comfort? sheets, pillows, towels
4.) What will you need for entertainment? television? dvd player? i-pod? portable radio?
5.) what will you need to enjoy the RV? bbq grill? (make sure it fits in space available) / folding chairs (we use the $5 Academy ones)
6.) What will you need for fun? bikes / balls to play with if you have little ones / etc
7.) what will you need for sanitation? hand soap / paper towels / mop, broom and bucket / cleanser or clorox / black tank treatment / dish detergent
8.) what clothes? don't forget swimsuits...we have 'trailer' clothes that are used only while camping and come out only for washing
9.) RV itself: white fresh water hose with (if you want) an RV filter on one end and an (important) pressure regulator on the other / sewer hose with fittings / gloves to use when dumping tanks
top off the propane tanks before your trip
don't forget any meds any of your family may need
if bringing pets, adequate food...water and food dishes
hope this is useful! (FYI we travel with our five year old...his most often asked question is, 'Daddy? when are we going to the trailer again?'...it's his very favorite thing to do...just since the start of school for him in fall 2011, we've been out for 13 weekends..soooooooo, have fun!)