Melissa,
Honestly, tell Tim that Jake and I feel very similarly to him about everything being a gift and incredible, etc., but this is just one area where I would ask, nicely. I agree that if they are going to be able to do anything about it, they might need a day's notice because they may not have inventory there at the moment. You could even suggest that you could use part of your expense check for the upgrade, if it makes Tim feel better (I'm sure they would say no, and/or Avis would upgrade you for free if they can, but at least you can tell Tim you offered). Explain about the wheelchair. We have a minivan rented, and I bet it's done purely based on family size. I'd bet your wheelchair makes your family's size needs as great as ours, with just one more child.
Please ask. And again, we are a family who likes to "make do" with whatever people in their incredible kindness have offered us. But this is not a preference issue, it's a practical/logistical issue re your ability to get around.
(On an unrelated note, still no packing done. We have all tried on bathing suits and found some that fit -- although my oldest is insisting on wearing ones that my hubby thinks are DEFINITELY too small -- at least she doesn't have enough of a figure to start people talking.... I spent last night until two am doing bills and writing letters to labs/collections agencies, etc. about insurance nightmares. Had to get done. It will be a LATE night tonight. So if you want to post at 2 am EDT, I will be up up up!!!!)
Good luck with the van issue. I understand completely where Tim is coming from (I'm that way and my husband is even worse -- they should get along). but this is something practical that should be addressed.