Dig down around the end of the tank where they now have the dump fitting and see if there is enough room to cut it off and connect horizontally. You can use rubber hose connectors that will slide on the pipe and connect it together.
Iirc you need to maintain about 1/8 inch per foot of drop on your horizontal run. In 40 feet that equals 5 inches. Depending on how they did it you may just cut their 90 degree pipe off near the tank and run horizontal over to your RV then use a 90 to come out of the ground with a threaded adapter just like an RV park dump.
If you are unsure of when the tank was last pumped you might have it done before you move onto the property. All jurisdictions are different and some may want you to get a permit if you tap into the system. Some may want you to get a septic inspection which amounts to the septic guy running water into the tank. They measure the water level and watch that it weeps out into the field. If there is a problem with the field the water will start rising in the tank and cover the exit fitting which goes out to the field. Some tanks have one cover and some two covers.