Can propane gauge be installed inside?

Yes, it can be done. Note that you want to read the liquid level, not the gas pressure. You would need to replace the gauge on the tank with one that includes a sending unit. One type is held in place with screws, and the other just snaps on. Which one you need depends on which type you currently have. The tank does not need to be empty to change the gauge since it's magnetically coupled to the float unit inside the tank. For inside the coach, the cheapest way to go would be an accessory fuel gauge from an auto parts place. The 0-90 ohm GMC style gauges work with the most common LP sending units. The GMC gauge will have wiring instructions with it. All that's needed is a 12-volt source, a ground, and a wire from the sending unit. The other wire on the sending unit can be grounded at the tank. A small switch can be added to turn the gauge on and off if desired. Amazon and/or eBay should have everything you need.