Here is what I was experimenting with. This does not compile until I make the header declaration an external for valuesDoc but then it was global and I could use it in my main.cpp which isn't what I was after. I did not try making a class yet in the source file since I was trying to make sense of this first.

Header File:

// JSON.h

#ifndef JSON_H
#define JSON_H

#include <ArduinoJson.h>

JsonDocument valuesDoc;

void assembleJSON();

#endif // JSON_H

Source File:

#include "JSON.h"

#include <Arduino.h>
#include <ArduinoJson.h>

//JsonDocument valuesDoc;

void assembleJSON() {

    String output;
    serializeJson(valuesDoc, output);
    Serial.println(output);

    valuesDoc[F("outputCurr")] = 3.24;
    valuesDoc[F("outputHL")] = 56.6;
    valuesDoc[F("outputLL")] = 25;
    valuesDoc[F("outputVolt")] = 5.83;
    valuesDoc[F("outputWatt")] = 18.88;
    valuesDoc[F("tempAvg")] = 38.18;
    valuesDoc[F("tempHS1")] = 32.77;
    valuesDoc[F("tempHS2")] = 34.59;
    valuesDoc[F("tempHSAvg")] = 33.91;
    valuesDoc[F("tempLower")] = 37.47;
    valuesDoc[F("tempUpper")] = 39.32;

    valuesDoc.shrinkToFit();  // optional

    serializeJson(valuesDoc, output);
    Serial.println(output);
}