A 1990 with a title with no problems in that length range would only go for about $10,000 or less.
Think what you are paying.
Is the frame bent? We do not know. So you could be buying tires frequently or fighting the steering wheel every mile you travel.
You say the rig runs good? How do you know?
You will have a hard time insuring the vehicle as some companies will not touch the risk. SO you may end up paying a premium for the rigs insurance even if only liability.
A telling item to me would be how long the current owner has had the rig. Did he get taken and is unloading it on you?
You should be looking a new tires before traveling with the rig. Tires over 5 years old should be treated with great suspicion regardless af how good the tire looks. RV tires are NOT REPEAT NOT the same as car tires RV tires sit in the sun for long periods just baking away. This and the non use for long periods dry out the tire and MAKE IT FAIL AT SPEED. Lots of damage can ensue. Any tire over 7 - 8 year old should be changed!!!
Now as for other factors with a rig that old you will be excluded from some RV parks. With 3 dogs you will be excluded from other places.
This is a bad buy which might leave a horrid taste in your craw souring you on a great lifestyle.