I don't think it will be a big problem; how cold you can reasonably go depends on the design of the motorhome. If you have enclosed tanks (typically with a furnace vent outlet in the enclosure), you can go a lot lower than the high 20's with the heat on. If the tanks and plumbing are exposed to view underneath, it's much less cold capable. I think most motorhomes have enclosed pipes at the least.
The furnace does use a good bit of propane, but around 30 degrees outside it shouldn't need to work overly hard. Definitely keep an eye on the propane level and refill as needed. I could probably go a week or two under such conditions in my motorhome before needing refilling, and it doesn't have exceptional propane capacity or insulation.
Be prepared for some draftiness and condensation on the windows. If you have an electric camp site (and I'd suggest that you look for them), running one or two space heaters, preferably on low, in the cool areas can help a lot with making things more comfortable and with reducing the propane consumption somewhat.