Cold Weather & De-winterized RV

28 is nothing.

I use a thermo cube connected to an electric heater. The Thermo Cube is a Thermostatically Controlled Double Outlet that plugs into any of your 120 outlets - it will turn on your electric heater when the temp gets close to freezing. Inexpensive way to protect your rig.

For many years I would winter camp in my RV in very cold temps and would simply wrap certain parts 'exposed' to below freezing temps.

I mostly agree with others , elec heat cant hurt but for the most part you should be ok at the temps you have described .

Some plastic fittings and valves are the first to crack depending where they are located.
Example: my water pump and valves are located inside an exterior baggage compartment but its enclosed inside a heated box , heated by a direct duct off the furnace.
If you have a setup like that, then you might want to run the furnace to be sure or wrap it in a heavy towel.

Keep an eye on exposed low point drain petcocks if they are exposed. If so, you could simply wrap them with something to insulated them. You have to keep the direct wind off them.

Place a bottle of water on the counter next to an outside wall and keep an eye on it. If you see it begin to freeze, turn on the furnace heat.

28 will be fine. Do run a heater and open the bottom cupboard doors.

Run a small electric heater, probably dont even need that.
Your Rig is insulated and wont get as cold as surrounding area.

Can't hurt. I sometimes run a 1500W electric heater in mine, off shore power.