Enlisted

As a different approach that I've found success with is using the engineer squad. They have multiple ways to earn xp, depending on the situation or your playstyle. The first is by getting kills. This can be done with the primary weapons, bolt action or semi auto rifles, or by building and using unlocked emplacements, AA or AT. The second way is by capturing objectives. This can be made easier by the fact that you can build defenses like sandbags and barbed wire. And finally, you can get xp by building rally points in strategic locations that your team spawns on, just don't forget to protect it!

If you are still having trouble by the time you get a golden vehicle ticket, most likely from a battle pass, you can redeem it for the T-50 as Allies in the Moscow campaign. You are basically invincible to enemy tanks right now! I played an entire invasion match with the same tank, Moving all the way to the last objective to capture, before running out of ALL ammo. Then I hopped out and mowed down some Defenders before being sniped. In the end I got over 100 kills, all the vehicle kills, almost 500 assists, and only lost that one squad. I leveled up my tank squad 11 times in one game!

Just use your main squad that has the highest level. Also make sure the join any battle option is set to off in the pre game lobby. Find a safe area close to the main battle and use the double tap X command so your squad stays put.

I recommend leveling the Assaulter class you unlock early on, SMGs are a very good way to earn plenty of kills and killing is probably the best way to earn XP in this game.
Leveling works in a way that the squad is rewarded for the actions specifically done by that squad (killing, capturing, assists, etc.) so if you really want to focus on a single squad, you could try dying/suiciding with your other squads in the match, so you can cycle back quickly to the squad of your choice.

Buying premium, obviously, will make things go twice as fast. Your call.