Looks like maybe 100' or so to the truck. At 70mph, that's about 1-second of travel time. The recommended safety margin is 2-seconds. Yes, people drive that close, and no, they really shouldn't at speed. That's why they came up with the 2-second thing...people couldn't remember x car lengths at one speed, y at another, and so-on. It's easy to estimate 2-seconds by just counting when the vehicle ahead passes some reference, and then how long it takes you to cross it.
Back to the OP's question...when your battery charge is getting quite low, the car will start to limit how fast and how much you can discharge the battery. The higher your speed, the steeper the grade, the more load you have in the car, the more things you have on, you're falling back on the REx to try to maintain the SOC...it's not going to be able to. It certainly can if you reduce the load (speed is probably the biggest, easiest thing to do that). If you don't, the SOC will continue to decrease, and may leave you sitting by the side of the road while the REx builds some charge back up, assuming you have any fuel left. Obviously, every road is not uphill, and the load changes considerably when you go back down, but speed is a big power hog. At some point for that day, your tire pressure, the temperature, your speed, you will max out the REx's capacity, and will be also drawing down the internal battery. With the original REx, that's around 70mph on flat ground. It appears you may get a little more with the newest versions, but not a huge amount. Just the difference in drag between 70-75 is an increase in drag of about 15%. That makes a significant impact on your overall maximum range. 70-80 is almost an increase of 31%.
If you can live with those restrictions, you can keep driving the i3 as long as you keep filling the gas tank about every hour or so, depending on conditions and when you're assured of finding an open station. If on an interstate, depending on where you are, you may need to stop at shorter intervals to ensure you can find an open station. Depending on where you live and drive, if you're lucky, you'll find an operational, open CCS unit. But, instead of 3-4 minutes to get out and fill the tank, you're talking about more like a half-hour or more, depending on your charge level. Find only an EVSE, you're talking about 3-4 hours, or maybe more, depending on the size of your battery, its SOC, and the capacity of the EVSE.
This is why I don't consider an i3 all that convenient for a road trip. It's great around town, and the REx can certainly help if you decide to take a detour, but it would not be my first choice on a long trip. Doable, yes, desirable, no.