We camp in an HTT down to 19 degrees with no issues, with day time highs in the mid 30s or warmer. We have an open underbelly and do not have an electric fireplace. We use a 1500W (5120BTU) ceramic heater and sometimes we use the 5K BTU electric heat strip in our AC unit. If both are on and both tent ends are open we have woken up to the HTT being 80+ degrees.
As for your MH. I expect the fireplace will do a fine job down into the 20's. Couple things I would recommend. Have a small fan to help distribute the warm air through the MH. Second I could consider picking up a small ceramic heater, just in case the bedroom is a little cooler than you like it.
As for the MH build. Your tanks are exposed like ours. I have never froze them up. We do not have heat pads on ours. I suspect you do, turn them on. The tanks are large and have a huge thermal mass and take a long time to cool off. They will be fine. Bigger issue is your plumbing. I suspect the pipes primarily run under the floor and are fully exposed to the elements. The pipes do not have a huge thermal mass and will freeze first (mine are all inside the TT). There is a true risk of them freezing. A few people have added heat tape along side the piping and then pipe insulation. If the temps are in the upper 20's I would not be concerned, but mid 20's or cooler, the radiant heat off the MH may not be enough to keep them from freezing. Skirting would definitely help.
Good Luck, please report back on how it goes and what you have learned.