[Big_Ben]

I am planning on taking a month long cross country road trip this summer with several friends as one last adventure before we all turn 30. The route we are looking at taking is just over 7k miles but I imagine with local driving and side trips that the entire trip could come close to 10k miles in total.

There are 5 (possibly 6) of us going and we all either drive small cars or big inefficient SUVs. In addition to not having enough space the depreciation on a car after adding 10k miles just isn't worth it for any of us to drive our own cars so we are looking at rentals. Based on size requirements it is pretty much required we either get a mini-van or SUV with a mini-van being the most fuel efficient but even they get fairly poor mileage. We checked the rental rates for cities close to our starting point (Lexington, KY) and found the best deals on monthly rentals in Louisville.

Anyway, I'm posting here to try and figure out how to maximize our MPG and reduce our overall fuel cost so we can put that money towards more rewarding expenses. Considering the difference between 14-15 mpg and 20-22 mpg could be a savings of $1000 over the length of the trip it is definitely worth it to invest a couple hundred in modding/tuning the car in the beginning. So I'd like everyone's thoughts on the mods I can and can't do with a rental. I'm willing to devote quite a bit of time into reading & prepping beforehand and willing to spend a day modding/testing at the beginning of the rental.

Requirements:
Has to be 100% reversible
Risk of damage to the mini-van should be almost zero
Can take up to 1 full day to do
Can't require buying model specific parts just in case that year/model isn't what we get
Willing to spend $100-300
Needs to increase efficiency by 10% to be worthwhile

My thoughts:
1) Fold in passenger side mirror (possibly drivers side on straight highways)
2) Remove antenna and store
3) Remove wiper blades and store?
4) DIY tire covers attached using new lugs with screws drilled
5) Use of scangauge (not factoring this into cost since it can be useful to me afterward)
6) Inflate tires to max sidewall
7) Quick partial grill cover (anything better than duct tape to attach it that won't leave adhesive residue?)
8) Tape over seams in lights, grill, front bumper, maybe even hood, etc (again anything better than duct tape that won't leave residue?)
9) Oil change - will definitely do at least 1 during the trip but unsure if I should invest in one before we start to make sure oil & filters are clean

Anything I'm not thinking of? Any of those sound like a bad idea?

PS: Efficient driving will be a top priority but I'm hoping to find a few mechanical things that may help as well