Gift ideas for my non-materialistic in-laws?

Hahahaha. Your FH's parents sound like my FH. How much are you looking to spend?

I would recommend something they can DO rather than something they own. For example, maybe tickets to a concert, zip lining, whitewater rafting, an art museum, etc.

My in-laws are similar (in my case they are divorced). DH's father also spends a lot of time (with his wife) in his RV but have a townhouse as well. We have often gifted him gift cards to Loew's. He can use it for the townhouse or something for the RV. For both his parents, we got them a nice picture from the wedding, you could do that. A picture in a frame is easily movable and won't take up too much space. Or I have seen photo blankets which would be nice. It would be useful and personal and therefore should be more appreciated.

I second @Elizabeth's suggestion. An experience gift is a great idea. I don't know where you are but maybe something like a day trip or tickets to a show or whale watching...

I'm doing food. I'm making scones the night before the wedding, and I'm hoping to also make chocolates. I definitely think food (it's not hard to make something gluten free, although I wouldn't go with scones) or experience gifts are the way to go. My in laws make significantly more money than we do and have more assets, so they prefer handmade stuff or inexpensive trips together (small theatre, 4x4ing, etc), to either material stuff or more expensive tickets for whatever, so although the reason's different it seems a similar situation.

Also, you could go camping with them, since they're into RVing and stuff.

In addition to what the others have said - a nice framed photo could go a long way, too.

Have you guys got them an album of the wedding photos? They would probably love that! I would also suggest getting them a gift certificate for a hotel chain, like the Marriott. That way when they're off in their RV they can "splurge" one or two nights and stay in a nice hotel along the way they are driving. Or maybe even a gift certificate for a nice dinner out!

If they like to work on the house and spend time in the RV, how about gift cards for Lowes/Home Depot for the house; gas cards or Autozone cards for the RV; or something that would be helpful along the way.

You could gift them a cleaning service, an "experience" like others have mentioned, or even a food basket with high quality gluten free foods and the coffee you know they like.

I agree with the "experience" gifts. I think I am getting my parents a weekend at a nearby all inclusive resort. Maybe hot air ballooning or good seats at a baseball, basketball or football game for your in-laws?

I would go with something they can do. Maybe check groupon or living social for experiences.

This is coming from someone their age and somewhat similar in approach to "things". Do they have a tablet? I live far away from some of my family and FACETIME is my best friend. When you have kids, they're going to want to stay in touch and see those babies when they are on the road. I know, I know......... I didn't think I needed one either, but even our generation can get hooked in the blink of an eye once we're exposed to a little technology. Ignore their skepticism at first and check back with them in 6 months. Preload the apps you think they might want... Believe it or not us oldsters LOVE Pandora. A cloud based photo-storage would be great for the pics they take on the go. Game apps like Candy Crush, Card games, hidden object etc are a great way we keep our brains up to speed. Give them a certificate for an hour of training from you or a family member every other week for the first two months. I have yet to hear of anyone regretting having one or not using it ... a lot.

Other options......... give them a year of Netflix or another streaming option. Amazon Prime is a big hit with my generation. We hate paying for shipping.

Call their RV store of choice............ ask them to invite them into create a wish list.

If all else fails, please consider spending an afternoon with them going through old pics, offer to scan their favorites to a CD or flashdrive for them and then create a photo book on snapfish or other photo service.