How about sheets. I love my flannels but they are a mite expensive. Find some inexpensive plates, cups, flatware, pots & pans, potato peeler, cutting board, salt & pepper shakers, wire strainer. If you go to the right places they are reasonable and I need to have a set of everything in both the stick house and the RV. Nothing is worse than realizing what you need is in the other kitchen. A small plastic pan and drainboard for washing dishes. A set of towels for the bath and kitchen.
Outdoors you need a comfortable, lightweight chair for each of your parents. I also like a cushion that covers both the seat and the back, it keeps the dampness in the evening away from my back. An indoor/outdoor carpet and a doormat so every piece of dirt on the campsite doesn't end up on the floor of the brand new RV. And if it doesn't come with a vacuum system a lightweight vacuum and dust pan and brush. Extra hoses includng sewer hoses and HEAVY duty electric cords.
Hope I haven't overwhelmed you but those are the things I bought for our 5er. I still find there are things I didn't realize I need but I think I am almost there.
Sara Lou