Good advice so far. If I were you, I'd start researching:
1.truck campers. Even just research new ones so you can get idea of what they're about, configurations, etc. That will give you background info. And, will owning a truck be conducive to your daily lifestyle, not just for vacationing.
2. very small towable RVs, like RPods, Microlite, etc. You can do searches for "small lightweight RVs" and get websites that actually list different brands to explore. I would start w/ looking at RPods first just to get an idea. Little Guy is also making Tabs now (discontinued brand brougt back, similiar to RPods). We ordered a small Livin Lite VRV toyhauler; so I get the minimalist approach you're taking.
Personally for what you're describing I don't know if I'd go with a pop up, just because setup, etc, doesn't sound like what you're looking for. (nothing against them, we almost got one but didn't due to cargo needs).
So are you saying you want to take the RV to state park campgrounds, park it & go hiking at the parks?? (this is our style) State parks usually have campgrounds that are easily accessible. Or are you talking more you want to "boondock" at State Forests, etc; where the campsites are more backcountry, undeveloped, and you have to go down forest service roads to get to them?? Because the forest camping I think would require lower profile for tree branches overhanging roads, maybe offroad tires/package for dirt/gravel/rough roads, etc. We've not done this, and I tend to think of this more for tent campers, backpackers especially (we're "car campers" where we park at the campsite then set up tent).
ANY C or pickup camper will be thirsty unless it's a Sprinter and then it will be expensive.
You didn't say what you have for a vehicle now because if it can tow that can help in a decision. There are some small trailers that may fit your need.
My "other" RV is a 21' Chinook which can get in just about any campsite, drive down narrow roads, and park anywhere a dually full size pickup can. 9-10 mpg though. Yes, it has a shower and you can hunker down inside if the weather goes south. We usually cook outside anyway but the Chinook has a micro and gas range which can be used. Wet bath which means hot shower and pooper in same location. Big downside of Chinooks is they must have thought their demographic was all retired couples that didn't get dirty. All of them are fully carpeted which sucks (for us outdoors types). It is not uncommon for smart people to strip the carpet and replace it with something sensible.
work on narrow side roads and well maintained dirt roads (like barlow pass)
seat and sleep two
comfortable sitting space for two to survive a day in the rain
hot water (shower not required but would be nice)
tall enough to stand in and change
operate in three seasons (be able to function slightly below freezing in an emergency)
be heated or heatable
You can easily find pop-ups that will fit your bill. Having lived in PDX, I've camped along Barlow Pass a few times. An 8' or 10' box pop-up would suit your needs. Easy to get them with a furnace, 3-way fridge, stove, water tanks & electric pump, and hot water heater. Something like the Fleetwood/Coleman Santa Fe (and many others). If it has a straight axle, consider doing the axle flip (over-slung/under-slug conversion) for added clearance. My axle flip added 5" overall and I'd be confident taking it on Barlow Pass (but I used to take a Ford Taurus sedan on BP).
My family (4) have weathered a couple of Pacific NW storms, plus many Rocky Mountian rain/snow/hail storems in ours. Last May we spent a grey weekend at Mt. Saint Helens, and beautiful weekend at Beverly Beach SP. An awning helps add some dry(er) outdoor space.
I'm 6'-1" and have no problems with the inside height. With the 4 of us, we use the bunk ends and leave the dinette set up. Only time we used the dinette to sleep on is when we had 8 (4 adult, 4 kids) using the trailer for a weekend at Rocky Mountain NP.
I've used my pop-up in freezing weather. There are water lines exposed underneath so I'm cautious about emptying them when not in use. If it's only dropping down to 25-degrees or so overnight I've not had any problems with them freezing. If the temps are cold for much of the day, I winterize the lines & tanks and use water jugs instead of the on-board system.
The furance works well. I keep a small poratble electric heater in the trailer as well and use that when I have electric hookup. If it's really cold, we use both.
Plus I keep my pop-up in the garage so I don't have to pay any storage fees and my HOA can't complain. And my truck's gas mileage only drops by 1/2MPG when towing the trailer compared to empty (and a few times it has increased when towing!).
The above suggestions are good, and it will be managing trade-offs:
If you want a vehicle for a daily driver and can afford it, a 4x4 class B (Roadtrek has a 190 Popular with 4WD, think it is the Quigley conversion) that would be good. It may be a bit cramped, but you can turn the beds into a dinette area. The downside of a class B is that it can get cramped, and switching the bed to a dinette can get old. The upside is that a class B can go anywhere but a parking garage, and doesn't look like an RV.
Another option is the pickup truck/TC. Some of the TCs (the Chalet three slide comes to mind) can be decent, offering a dry bath.
If you want a dedicated 4x4 RV where on-road performance isn't as big an issue, I'd recommend a 4x4 class C. Phoenix Cruiser and Tiger Motorhomes are good choices, because they have more space than a class B, and offer more features, like a larger generator, dry bath, etc.
For off road trips and hiking, I'd look into some of the following features that may come into handy:
An inbuilt Onan generator, be it LP gas, diesel, or gasoline. This way, you have A/C and can recharge batteries at the flip of a switch. This way, if it is hot and rainy, you can keep the A/C on.
An external shower. This helps if you are really grungy, and don't want all the mud to wind up in your bathtub. It also helps with hosing off shoes and such.
A winch, either portable, or mounted on the frame.
A solar setup and an AGM battery array for the house batteries.