Outpost 2.
As far as I know, it's the only RTS where you actually kept your stuff between campaign mode maps. Admittedly, most of your buildings were abandoned due to narrative reasons*, so it was mostly a matter of resources and, more interestingly, the tech tree. Due to how it was structured in campaign, you couldn't get too far ahead, but it was enough to end up with a very real advantage if you went into each map fully loaded and as advanced as you could be, as opposed to just ending each map ASAP.
*Plot is kind of awesome in a scifi B-movie way: They were trying to colonize the planet and released (I want to say it was intentionally, but that'd be TOO dumb...) into the wild a microbe that would actively seek out and split all water into gaseous O2. Oops. So most of the game is spent getting enough resources to keep outrunning the wave of death.
Warzone 2100 https://wz2100.net/ uses the same mechanic with persistent units, as well as bases that you gradually develop over several missions before eventually abandoning them.
Also the Earth 2150 series, and it's many expansions did this, not sure about that sequel set on Mars tho, that one's kinda fuzzy. And both Homeworlds had you with a persistent fleet that followed you around, think it also scaled the difficulty depending on how well you were doing in terms of tech/fleet size, so staying around to farm the map wasn't always the best idea.
Yep, Earth 2150. I still remember how I used up all of my resources and units in one mission only to have them all hacked (thus losing them), so I reloaded and failed the mission on purpose. As a consequence I got a really hard mission which I can only assume was supposed to be unwinnable (and losing it caused a game end), but after like 50 tries I managed to win and it had so many minerals on it it slingshoted me half way through the campaign (the whole campaign goal was to send 1 mil res to orbit).
And to add to the topic. Original War, a blend of RTS with RPG. you had a fixed amount of people, and everything required people. Research? Tank production? building a turret? having that turret actually shoot at enemies? having a tank drive around? gathering resources? All of it needed people doing the job, and if someone died there was no replacement. And you had on average maybe 10-12 people per mission, each with their own stats.