Our dog is marking in the house - any advice?

First, have any medical issues been ruled out?

Second, do you know if he was reliably house trained previously?

Third, he's a scent hound. I've never had one, but my gut is that is the real issue here, combined with your home being the first place he's lived post-neutering. Marking can be a pretty common behavior in response to scents and your fella can smell *everything.* I'm afraid I have no advice because I don't have any experience with scent hounds, and though I'm not sure that's the underlying issue, I strongly suspect it's a big contributor.

I'd look for some online rescue groups for plott hounds or other scent hounds and ask members if they have any advice. I'd wager you're not the first to deal with this.

I had a boxer that did and basically your screwed. Brings more pets into the house is only make it worse.
If he hasn’t been neutered, getting him neutered might help. My boxer had been neutered.
He is marking is territory, any new scent will cause him to remark that spot.

Unless you are soaking the carpet, including the pad, down to the subfloor you won't get rid of the smell. 

When Emma was a puppy, she would only pee in the basement and she would pee even if she had just been outside.  I did some asking around and found out that the previous people had kept their dogs down there when they were gone all day at work and the dogs would pee. At that point, the odors were deep into the concrete subfloor.  I was vigilant about keeping an eye on Emma when we were downstairs and would scoop her up the second she even looked like she was squatting.  She was probably about 6 months old before I would let her loose down there and she was never unsupervised.  I fear that you have found an old pee spot in your house.

Depending on your house layout, best bet might be to block off the places he's marking and not allow him access. We found some nice decorative folding gates at Home Goods.

Absolutely nothing wrong with that. Just be sure to talk with a trainer on how to do that properly with an older dog. You don't want them to view it as punishment, but for the crate to become their den/home. Done correctly, it can become a safe space for the dog to retreat to, even on their own.

We have a 6 month old German Shepherd/Bernese Mountain Dog mix. We use doggy diapers whenever he goes in a vehicle because he pees every time. I think he has car anxiety and am hoping he grows out of it. In the meantime, the doggie diapers are allowing him to go for car rides.

I agree with this. Don’t give up yet, but I agree with the “back to basics” approach. Treat him like a puppy in that you watch him, wear him (leash attached to your waist), and crate him when he can’t be closely watched. You need to catch him BEFORE he marks and get him outside every time. I’d probably do some mild scolding as well, but definitely nothing physical. It’s harder and more work to retrain a habit, but it can be done. 

I adopted an elderly non neutered Pyr who had only lived outside…and he marked a ton. And he was the size of a horse so his “sprinkle” was a lot of pee. I despaired if ever fixing it, I mean he was 10 for gosh sakes.

But, we did! I’d leash him to me or keep him right with me (because he didn’t move a whole lot 😂) and every time the leg went up make a disapproving noise- and for a Olaf most important- an exaggerated frown face of upset. Olaf thought I hung the moon, making me happy was his life goal. And he learned! If he actually peed I did nothing, just quietly cleaned it up not looking at him at all, I only did the sound and frown if I caught him before he did it. And he stopped, in a week! He never could get neutered.

Olaf was an exceptionally smart dog, but the tactic has worked with other foster dogs. Some have had to wear belly bands though to be unsupervised in the house, the terriers especially were just impossible. Some dogs who stopped in my house would immediately mark at others houses/stores etc so had to be very vigilant. It’s a tough behavior to break and neutering IMO doesn’t even help much once it’s started.

Give him time. You can train the behavior with a belly band on, the dog doesn’t notice nothing is coming out and it will save your floors.