We want to hook our holding tank up with her septic and wanted some input from folks who have, cautions, problems etc.
You have to decide whether to leave the hose in place or just deploy it when dumping. Another alternative is to bury a pipe, if you're going to be there all summer.
If it's nearly level but you don't have at least 1/8" per foot of fall then you can use the "sewer solution" which uses a water jet to move things along. I have one and it works OK as long as no one flushes a tampon. They're around $100.
Otherwise if you have enough fall you can use a regular "slinky." You may need or want a ramp of some kind. You can buy them or make them from lumber.
Also is there some device that can hook semi-permantly with the 3" "observation" hole in the top of the septic tank that will prevens odors from escaping?
Yes, the "sewer solution" comes with one, or you can get them for use with the "slinky" from RV places, tweetys.com, campingworld.com, and the other usual suspects.
Be sure that the "observation hole" isn't also a vent. In some septic system installations, where the observation hole is not capped, it must be left open.