Here's some other advice that might save some money. Look into the America the Beautiful passes. Here's the site for more info.
http://store.usgs.gov/pass/index.html
If your father-in-law has handicap tags, make sure you bring them.( My husband is a disabled veteran & we have the license plates.) His permit tag will allow you to camp in the handicap sites. They're often closest to the bathrooms and usually some of the longer sites. You'll appreciate that with your 34'.
The ABP for seniors and permanently disabled can get a lifetime pass for free. Check the limit of guests his pass would give discounts to at all Fed. Gov. recreation. Some places you might get to visit free or stay for half price, like the USAMC campgrounds. Your kids might be young enough to get in for free or for a smaller fee.