Thank y'all for your answers and input, especially on camping and driving recommendations. Let's see if I can address most of the points.
We can definitely work around any closed roads, but I guess some passes really make for a nice drive especially Trail Ridge Rd (maybe also Old Fall River Rd). I'll see which roads/passes fall in this categorie and then check out when they open this year, to get an idea.You have picked a nice time of year to travel, away from the peak season and as Spring is moving to summer but you are right, it could be cold at elevation and some passes could still be closed, the Trail ridge road in RMNP and Tioga Pass that crosses the Sierra's are 2 examples but there is just no way of knowing until you are travelling. It's something that can be worked around in any case if need be.
Quickly ran the numbers and the upgrade from fullsize car to SUV is ~600$ and another ~$1000 for the fullsize elite SUV (for 8 weeks). This already includes gas estimations for ~5500mi.If you are renting a large vehicle to save on camping fees it's a false economy really. The vehicle will be more expensive than a mid-size sedan in rental and fuel costs as well as the added time and miles of finding back country roads and Walmart's etc. When visiting National parks camp in them, there is nothing like being there amongst it all.
The idea was not to save money on camping fees, but I did a 2 1/2 week trip with a campervan a couple years back and really loved the freedom of only needing to have a rough plan for 1-2 days ahead. No early reservations, almost no "we need to drive as far as X today for our motel". Gave us lot's of freedom to put in many things (hikes, sights, scenic detours) when we stumbled upon them without worrying about arrangements. Finding a place to sleep was never a problem.
Yep, we're aware of this. Reducing this risk is one reason why we lean towards an SUV type rental. Still we won't be doing e.g. the alpine loop 4x4 sections, no matter if we end up getting a 4x4 (which is a total coin toss).Also note that going on the "Occasional dirt road" will almost certainly violate your rental agreement T&C's and would make you responsible for any damage or recovery fees that may occur.
.I rarely think it's a great idea to plan to sleep in a rental car/van/suv. People find a way to make it work, I guess, but there are a number of issues. One, you'll never know exactly what make/model you're going to get until you show up at the rental counter, and some models are much better than others for sleeping - some don't have seats that fold flat or have other obstructions that make it difficult to even lay down some kind of mattress. That's on top of the fact that cars just aren't designed to be sleeping cabins, so you've got issue with a lack of insulation and ventilation
All of the fullsize elite SUVs in the fleet I found so far have seats that fold fully down. Still there's a risk of make/model variation and maybe needing some workarounds for a nice sleeping setup.
The campervan rental on an earlier trip had a good sleeping setup, no additional insulation, no precautions for ventilation either and it worked well. Of course you need to keep some extra things in mind and it's minimal comfort but comparable to tent camping.
There's always the fallback version of tent camping but given all your concerns we'll definitely reconsider this as primary option. However apart from sleeping bags we'll need to buy everything we need after arrival and donate it at the end. So we want to keep the camping equipment to a minimum.
Dispersed camping was probably a wrong term for what I meant. The idea was to find a secluded place in the NF to park overnight. Remote trailheads, parking bays on forest roads or just a patch of gravel next to a small backcountry road worked well with the campervan and were never hard to find. Dispersed tent camping is not really an option, given that we still want to have our car close by.
National forest or state park campgrounds are always an option, given that they're usually first come first serve I have no idea how easy it is to get a spot later in the day. But from what I saw on allstays.com it seems as if there's a lot of these campgrounds and almost all of them seem to be accessible via maintained roads (sometimes dirt roads though).
South Rim, North Rim, Arches/Canyonlands and Yosemite have already popped up as "probably limited campgrounds that are filling up early" so we'd need a backup-plan or reservation here (or the calmness to accept whatever situation we get).Off-season in May/June doesn't really mean off-season at the national parks. North Rim (showers/laundry near campground for a fee) fills up far in advance. ...
For example, Arches is popular in the shoulder months because the summer is very hot.
As for rush hours in the cities, I already had the joy of multiple rush hour traffic jams in LA and Altanta. So we'll be trying to avoid those by hitting the road before 6am or after 8pm depending on how far we want to go that day.
After all thank y'all for pointing us in the directions of the many things we still need to consider and think through for this trip.
We'll let that sit for some days and then we'll see.