Try sittiing down when you are calm and re-read your own post. Try to read it as if you were one of us -- a stranger.
You end with:
"MIL is like a mother/ close friend to me. They're both great to us and our children. And they go above and beyond to be supportive of our life decisions. It's truly a blessing to have them in our life.:
That ending surprised me after the long description of how they are unreliable, don't ever come to your home when invited, etc.
So, can you possibly reverse your whole post, in your mind, and put your focus on those last few positive lines?
If you really DO love your MIL like you say, and if they are great to your children, you will need to accept that they are not, and possibly never will be, like "other" in-laws. They live in a truck. LIVE in a truck. With all the things that entails, like breakdowns and traffic issues and unreliable schedules. Is it ideal? No, not for you and your family. Should you just stop going to see them? Only if you want a rift and want these people whom -- at the end -- you say you love to be cut out of your lives.
If they lived in the same town and always refused to come over or were otherwise demanding etc., that would be different. But they live in a truck and they want to see you when they are in town -- you may not realize it, but you likely are their roots and their rock and their "hometown" in their minds. I agree, it is not considerate or thoughtful of them to fail to realize that with two very small kids it's not easy on you to haul everyone out to see them. But...you love them and they are a blessing and your MIL is like a mother/close friend. Your words.
Which will win? Your frustration at their lifestyle or your professed love and kinship with MIL?
Can you work on this? Can you and your husband -- it is really HIS job, they are his parents -- have a talk with them about their schedule and see if you can arrange visits that are planned farther in advance? Can you arrange visits where they stay in a nearby pet-friendly and trucker-friendly hotel? Such things do exist; they do not have to be under your roof to "stay" with you for several days. If you go out of your way to locate hotels where they can stay with their cats and park the truck, and can hook them up with those hotels and maybe even offer to pay part of the cost, think how much that would show them your affection and initiative and desire to see them.
I do agree that they ask a lot doing this over and over -- BUT it is every few months, right? Look at some of the Mamapedia posts about in-laws who live in the same town and who are nothing but demanding and nothing but critical. Be thankful that your in-laws are great folks and not living two streets away. So when they DO come into town -- maybe relax a little about it, and at the same time, take the situation in hand, talk with them about more scheduling than they do now, and try to find them accommodation so you can all be OK with the arrangements.