According to my computer mapping it is about 700 miles from Laredo to Mazatlan - no one day for me - ever. Then you have the Devil's Backbone at the end. Not a trip I would try - especially not for a newbie. Better bet is over to Nogales and then Puerto Penasco or San Carlos. Newbie needs to read all the Stickies at the top about permits and insurance and so on. Remember you need a passport to get back into the US.
Mexico has some good roads and many are getting better all the time. But you will rarely average 60mph or more. With one thing or another we figure on about 40mph average and hope to do better. There are more things to slow you down in Mexico. And finding suitable RV parks or campgrounds can be problematic. You need to do research on this website to review old posts that explain some things - and ask more questions and get more specific. On this forum you will find experienced people who are more than willing to help and guide you. But we do not all agree so telling us your style of driving could help to connect with those of like minds. In Mexico we rarely go 300 miles in a day - though we have and will again - it can be a long day depending on the roads. We are in no hurry and like to take our time seeing the sights along the way. We also never drive at night in Mexico - speed bumps - cows - horses - sheep - goats - and who knows what could be out there on the road. The next RV park could be a long way away so planning stops is important until you learn the ropes.
We still recommend this book . . . http://www.rollinghomes.com/mexico%20book.htm . . . but some would disagree. It tells a lot but is an old edition so out of date - but their website has updates and if those in the know would post on the updates it would help.
Another great source is . . . http://www.ontheroadin.com/ . . . where a great deal can be learned. We bought their road logs years ago and still use them daily - we do our own updates in the logs as we go - they are well worth what they charge.
Some travelers prefer . . . http://ioverlander.com/ . . . and it can have a lot of useful information though we have found many RV locations in Mexico are not listed - but it is getting better.
Mexico is not USA so their is a learning curve. Some people just go for it while others prepare ahead. The more prepared you are the more enjoyable things will be.