Installing a 2nd leisure battery

Twenty five years ago, on our first motorhome, I installed another battery along side the existing one. Then I coupled the two batteries negative terminals together. using an appropriate jumping cable. Finally, I fitted a quality clip on terminal on the single positive cable. This + cable could then be alternated between the batteries. Okay, so the system did lengthen off grid camping opportunities but it called for careful management of the charging (either when driving or when available on EHU). I also had to ensure that on using neither battery was ever depleted to that critical point but that I already had to do when I had just the one battery anyway. This system worked for some 5 years wirhout issue until we bought our first brand new MH which came with a second battery  'permanently' wired in and had its own inbuilt charging management system with a low battery alarm. However, even with this I've still had to keep an eye out so as not to reach that critical discharge point on either battery. Now the question is, as I'm soon to swap our van for a new one should I go that one step further and install a solar panel? Part of me thinks though that I could return to that system I used all those years ago, afterall, it wasn't that much of a pain! Any thoughts on the above or alternative systems would be welcomed, also, if I go down the solar route, what size would folk recommend?