Speedometer on HR Neptune

There are still some speedometer shops around that will be able to adjust yours to make it accurate.

An alternative is to get a dash mounted navigation system. These will accurately show your speed and mileage for each trip. Some can be programmed to show cumulative mileage for all trips.

Even the navigation app on my iPhone shows mileage and speed.

I would guess the speedometer is a more or less standard part for whatever chassis your motorhome is built on, and replacements stand a reasonable chance of being available from the usual suspects.

A GPS will give you very accurate speed measurements (or, at least, it can)—far more accurate than it could just by computing how far your position has changed over time. It's possible to compute velocity more or less directly from the doppler shift in the signals from the various satellites. The bear's suggestion is thus a good one.