CCWREW, I've moved your thread to the plumbing forum as there are many users that have looked at their tanks and valves and my offer some advice.
I would would not just start cutting into the coroplast but rather remove the screws that hold it in place.
You may be able to access the valves that way or at least see their location before cutting an access panel.
Peace
Dave
Dave, Not sure how to reply a post, but hope this is the way to do it. I am currently camping in Arizona from my home in PA. We have been here since January and are scheduled to leave on April 27th. Anyway, I went under the trailer and removed only three bolts from the under belly. I looked up into the cavity as best I could and see a lot of wires and insulation. I have stopped because I am feeling as though I am getting into something that I will not be able to handle here in the campground. So, I am going to have a mobile RV service do it for me. But thanks to all who responded. But, believe it or not, we had a "dusty devil" the other day that had so much wind force that it blew my trailer off of the jack pads and slide the trailer to the side about 2 feet. All seems okay, except that the brackets that hold the leaf springs to the coach frame have slightly bent to the side causing the shock absorber to be within 1/2 inch of the inside of the tires. So we have to have someone come out. They will have to jack up the trailer. So, question is, 1. In order to jack the trailer, won't it have to be hooked up to the truck so that weight is not put on the front opposite landing gear, and 2. Where are the jack points for 2012 Bighorn 3070RL. Generally, I think they would b e on the frame adjacent to the tire. I figure it only has to be raise enough to get pressure off the springs so that the bracket can be accessed. We are hope that they can be bent back to be perpendicular to the coach. If not I guess the trailer will have be flat bedded to a location that can cut them off and frabricate new ones.