Welcome to this forum. This site is great, I have learned so much from all the helpful folks here.
Your "new" trailer looks very nice. Having front and back doors will prove very handy I'm sure.
I'm sure others will chime in, but I will share what I know with you.
1. Deep Cycle Marine Battery - Our 298 holds (2) standard 12v batteries. I wanted to convert to (2) 6v, but unfortunately they are too tall. So I opted for 12v deep cycle from Costco.
2. Short Queen Mattress - For us, one of those memory foam toppers did the trick. We were pessimistic, but we are pleased with it. I think it was around $100-, and I had to cut it to fit.
3. Awning - Our awning is now 12 years old and probably its last season. There's a ton of info and YouTube videos on the subject, so I'm not too concerned about that job. I may even have Camping World do it if it's reasonable.
4. Skylight - Our original one has faded and is brittle - Ours also has "crazing" around the edges. On the last trip, we were in snow and rain, and it leaked some. I was able to mend it with silicone sealer. I did some shopping online, and there are lots of different sizes. So I need to be careful to get the correct one. I also want to change from the smoke tint we now have to clear. Just to get more light in there. Hopefully that's not a mistake for summer time.
5. Stabilizer Hitch - Our trailer came with an inexpensive "EX-Hitch" with equalizer bars and a single friction type anti-sway device. The hitch had a spot for an additional ball for a second anti-sway bar. So for about $60-, I added another. I could definitely feel an improvement. We use a 3500 Ram to pull with, so the tail wagging the dog wasn't horrible to begin with.
6. Replacement LED bulbs for lighting - We noticed immediately when we got our trailer that we could almost always hear the charger fan kick in when we ran some of the incadescent lights inside. So we replaced all 35 of them with LED retrofit type. It wasn't cheap, but definately worth it. Particularly if we're "dry" camping. I'm not sure if these are exactly the ones I got, but it's the same company. They work great, and are easy to install. https://m4products.c...aluminum-plate/
7. Locking Battery Cover - Never thought of it to be an issue. Nor have I ever heard of anyone complain that they've been ripped off.
8. Best cleaner for the exterior - When it's really grimy, we use good old "Dawn" dish soap and water. The roof is pretty much impossible to get back to white. I just scrub it as best that I can and follow up with "Protect-All". It's a lot of work, so it only gets that once a year or so.
Beyond that, I urge you to keep good tires on the trailer. Time more than wear is the big factor. Most seasoned trailer owners will suggest staying away from the Chinese brands. Although it's hard to find American manufactured trailer tires. Also, I discovered "E" rated 15" radials for very little extra money.
Occasionally our valve is sticky, and won't hold water. I take a "Scotch" pad and clean the rubber seal as well as possible. Then apply some plumbers or dielectric silicone grease to it. Seems to help a lot.
Probably not a solution to your problem, but I had to through this in...
I appreciate your concern of the parasitic draw provided by the new monitor. I like your suggestion of wiring it to the 12V feed from the truck, with the idea that the monitor will only draw power when connected to the truck. On my trailer, I have a digital volt meter at the electrical panel inside the trailer. When on shore power, it reads 13.8. When hooked to the truck it reads closer to 14. But, when I turn off the battery switch next to the batteries, there's no voltage at the meter. Even with truck hooked up and running.
I say all of this because there's different parameters and situations. Therefore, I think the only way to answer your question is to get out a volt meter that has continuity capability, and a simple test light. Start playing around to see where you can hook up your monitor for the result you need.
Perhaps I've made more of this than was necessary, but I think I understand what you're after. My guess is that it will just take some patience to work through it. Might be handy to have a helper with you.