Walmart or any RV parts store can sell you a gauge that connects between the tank and your inlet hose. As far as being able to read the proane level from the inside , personnaly i have never heard of a kit. I have installed the the gauges from walmart on my 5er, both tanks. they cost $20 each in the barbeque section.
Mark & Connie
2006 28RK Wildcat / 2006 Chevy CC 4x4 Diesel / 6 SPD Allison / RBW Slider
Pioneer in-dash navigation / Firestone airbags with in-cab controls /Prodigy Brake Controler / Honda EU3000 with wireless remote starter