Ok, you didn't mention other factors... so I'll bring them up, just in case:
- Your water hose... is it heated, and/or insulated?
- What about the city-water tap itself? Has it frozen up, or is it protected?
- Has the RV park itself turned off the water supply to protect their own equipment? Some campgrounds do this.
- Do you have water in your fresh water tank... and if so, are you able to move water via the water pump? This can help you determine if it's your RV pipes are the issue.
The coldest I ever endured was over-night temps in the mid-20s, but rose above freezing by mid-morning. All of my plumbing still worked, but I swore I'd never voluntarily test it again. I can't imagine enduring single-digits.
You mentioned heaters in the plumbing area. Not sure exactly where you meant there... but if you didn't mean your pass-thru storage / basement area, here's what I did.
First, I use these small electric ceramic heaters to keep my rig warm. I have 3 of them, and using them is much cheaper than using my propane furnace (especially when campground electricity is free!). They have adjustable thermostats, so you can set as desired & leave them on continuously... and are safe for use, even when away from the rig.
I use 2 in the living area... and when I know temps may possibly go below freezing, I use 1 of them in the basement.
In the kitchen area, some of my plumbing runs along the bottom of the cabinets (under the drawers)... so I open the doors, and the bottom drawers so the heat from the interior gets to those spaces.
In the basement, I have a hole cut into the wall where I place one of those heaters to blow continuous hot air into that cavity where the plumbing below the floor is. Not only does it keep those pipes warm, but has the added benefit of keeping your interior floor warmer for walking on too (especially the bathroom!).
Other than that, I dont know of anything else one could do without insulating each & every pipe on your rig.
Anyway, good luck... let us know what you find.