I had an X19H which is the next size down from the X23B (we loved our X19H but sometimes wished we had gotten the X23B from the get go, inside storage and Slide make a big difference, so you have made a good choice for a Hybrid).
I had an Equalizer WDH 1000 lb and it worked great, will be getting a new 1200 lb hitch for our 28BHBE. I had the option of adding the new electronic sway control when ordering our 28BHBE but after researching the system I decided while it was neat it was more electronic stuff to not work or go haywire. A proper TV and WDH setup and safe speed and driving conditions make for a good combination.
As for the enclosed underbelly I believe all it is is the black hard plastic sandwich covering which you could do yourself if you really wanted it. Jayco's website lists it as "Enclosed Underbelly" not a heated underbelly. I did not have an enclosed underbelly and camped in 30 Deg weather and had no tank problems. With the tents, which allow the camper to get cold inside, I would not think you would camp much in weather that having an enclosed underbelly would show any benefits mainly because the tent ends are the weak spots as far as thermal protection.
As for the underside getting dirty etc you can always clean it and you have full access to all the Tanks and Wiring which is nice. I ordered the Thermal Pkg on our 28BHBE more for the Roof and Extra insulation in other parts of the camper over the enclosed underbelly. I will miss not having easy access to everything.
In the end you have to decide what you want and if you think it will bother you later that you did not get it, then order a camper. That is one of the main reasons I did, I wanted to get a couple things that I might only need once in a while but I will have it when that need arises.
We have now gone to the Dark Side but will miss our Hybrid adventures.
'16 28BHBE / Elite / Thermal / Dual AC / 50 Amp / 12K Equalizer
'15 F250 Lariat CC FX4 6.2 w/ 3.73 / Air Lift 5000's / ARE Z Cap
'12 X19H Hybrid (Sold)