High temp but no overhating

I just bought a 1987 Ford Tioga Motorhome on a Ford E350 Chassis. When running the vehicle the temp gauge runs on the letter L in normal, which is just about to the upper mark in the normal range. When going up a moderate hill the needle only moves about 2 needle widths then comes back down when you top the hill, it never goes past the normal range. When I turn the heater on, the temp rises & will continue to the hot mark but as soon as I turn the heat off, it goes back down into the normal range, it does the same thing if I turn on the air cond. Any help will be appreciated. Thanks, Tom

I would talk to a radiator shop about having it cleaned and having a rod run through it then boiled out.
Use a water hose and disconnect both heater hoses flush out the heater core.