I’m a Spaniard myself and I can tell you the situation in Spain is pretty bad right know. I’ve never worked there as truck driver but I have friends who did and right know wages are not so good. Around 1500 euros per month doing national work. Expect 4 or 5 nights out. You could get in better companies If you had an “enchufe” which means someone working in the company passing your cv onto the boss.
If you had more experience and spoke fluent Spanish you could get something more decent but still difficult. Lots of experience drivers who speak good Spanish still have to put up with nasty bosses and bad conditions.
If you have more specific questions go ahead