My son has asthma that can be aggravated by mainly pets, dust, and smoke. We have a converter in our van in case of emergency so he can use his nebulizer. When we camp we camp at sights with electric. They are getting a little older and thought of expanding to other camping areas without electric. The ADA sights in these areas do have power. Personally, I do not see his asthma as a handicap. But if there was ever an emergency, I would like to have access to power so he can take a treatment. This happened a couple years ago and we ended up in the parking lot of a closed hotel, hooked up to a gazebo's outlet at 1am. We were there because the fuse blew in the car converter. Would it be wrong to reserve an ADA sight just for the power?
Nothing at all wrong with it, but to even further expand your camping options, you should consider a small generator. A Honda or Yamaha 1000 watt generator is very lightweight and small enough to go most anywhere. They are also very quiet and could be used anywhere in an emergency. With the generator backing up your converter in the van I would think you would be able to go anywhere without worry.
I would suggest using an inverter hooked up to your house batteries which you might upgrade if you felt the need. My wife an RN tells me that it doesn't take too much power, but you can check that out first. This would open up everything to you. Inverters up to 400 watts can be plugged into a cig lighter. Ours will run an electric blanket. I know campers use an inverter to run a CPAP machine all night on an inverter when dry camping. Good Luck.
You really need at least a small generator just in case. We were camped several years ago in the MidWest. A storm went through one night - we lost electricity to the campground and the access road was blocked for 3 days due to flooding and downed trees. I use an oxygen concentrator - without the generator would have been in a bit of trouble. As it was we just had a nice extended stay.
Asthma is extremely serious as you well know. My wife and son both have Asthma, but not to the severity of your son. We just make sure inhalers are present for them in the camper and vehicle we are traveling in. Asking for an ADA site for the power requirements would be a must IMO. Safety and Health are the biggest priorities!!!