Delamination generally spreads only if there is continued water intrusion (or remaining moisture from previous water intrusion), as I understand it. I may be misunderstanding. It does represent some structural damage; whether that is sufficient to cause significant issues is very hard to say without inspection etc. and to some extent depends on the construction of the RV in question. (It is possible for delamination to occur for reasons other than water intrusion, such as the natural breakdown of glues over time, but that is quite rare and essentially unheard of in relatively modern units. It's pretty safe to assume that water is the culprit.)
Most important, of course, is ensuring that the cause has been fixed; otherwise, things will continue to get worse as time goes on.
With a tight budget, you will have to deal with some compromises in an RV. While many people would immediately rule out something with the slightest hint of delamination, I would tend to just consider it as a negative--possibly a significant negative--and consider the unit as a whole. Typically the damage internally is rather more extensive than the outside appearance would at first suggest, doubly so if the trailer uses wood framing rather than metal. If the roof is soft or the walls show signs of sagging or other evidence of structural unsoundness, that definitely is grounds for walking away without further investigation.
Do make sure you budget for additional up-front expenses that almost certainly will come with any unit you purchase; things like tires, batteries, minor repairs, hitch setups, hoses, etc. can add up rather more quickly than one might initially anticipate.
I realize it's not what you asked, but for travel on a budget, a combination of tenting and staying in motels will quite possibly be less costly than buying and pulling a trailer around. There are, of course, tradeoffs in convenience when doing that; but $7000 plus gas savings by not towing can get quite a few nights at a Motel 6 or other modest but safe lodgings.
