Some meters can be installed vertically. Check with the manufacture.
You shouldn’t have rust at the gate valve (brass with galvanized won’t rust), just make sure you support the pipe below the valve before twisting, as you may break something below ground.
I would turn off water to park or a section of the park. Disconnect the union, the flex line, or cut the nipple between the valve and the tee. Next, try to remove the old gate valve and replace with a ball valve. Install the meter and then a tee. At the tee install a hose bibb and other side goes to the mobile. At the mobile you will find each space will be different, I doubt every mobile will be hooked up the same way.
I wouldn’t go under the tenant’s mobile. You don’t know what mess of plumbing you could find. Do everything exterior.
Good Luck